So a quick disclaimer: This is not an attested system of fate. This is guess work, unverified personal gnosis and linguistics. This is meant as a possible system one can adopt if they want, but it is ultimately meant for my own regional polytheistic practice. I know plenty of people out there are very touchy about innovation based on limited sources. However, it must be noted that the Gauls were not a monolith and that even the term Gaul is as broad as using the term Celt. Therefore I feel my system is just as valid as the next.
In Continental Celtic religion, there are very few certainties about the concept of destiny or fate. Often we look towards other Indo European polytheistic traditions, such as Hinduism, Heathenry, Greek Polytheism and Roman polytheism to fill in gaps within our own religion. Yet, the words and concepts that we do have don’t always match up.
However, with the words we have, I believe we can formulate a system of fate/destiny that is either recognizable or at the very least plausible for contemporary practice. The words we have for this are toncnaman, toncsiiontio, and lugjom. This is drawn from John Koch’s Further to tongu do dia toinges mo thuath [“Mi a dyngaf dynged it”], &c.
Toncnaman is theorized to mean fate/destiny where as toncsiíontío would be third person plural future verb described as swearing. *Lugjom is therefore oath, and connected to Lugus according to Koch.
In his paper, Koch describes how it was possible that the saying toncnaman toncsiíontío was formed from a possible tabu that involved the name of a very important Gaulish god (Lugus). To quickly summarize the paper, in order to avoid the tabu, the Gauls developed a way to say “I swear (a destiny) by the god of my tribe” without using the word *Lugiom and Lugus in the same sentence, while meaning the same thing. 
What we are concerned with however is that the words have the connotations of fate. We also have another element introduced then, the Dêwos Lugus. If Lugus is invoked in the making of an oath and destiny at the same time, this tells us an important idea of Gaulish fate.
“In general, it may be said that swearing with reference to the future invoking supernatural sanction is inherently close to destiny. Within the context of heroic biography and where the audience believes in verbal magic, there can be virtual equivalence.” 
What this says to me is that by swearing to this god of oaths and destiny (Lugus), one asks for supernatural sanction for this possible present or future to come true. But this doesn’t say anything about the past. It’s awfully tempting to look to Hinduism or Heathenry for a backwards engineered concept such as Rta or Orlog, but I don’t think it’s totally needed to do such a thing. There’s no outright mention of the past (as a concept) or a natural order that has and is happening all at once in Continental Celtic studies. So, we can either say it didn’t or doesn’t matter or we can figure out how it can fit in a constructed system which makes the past a passive element; still there and important, but not as important as what can and will be. I opt for the later.
As I’ve said, I believe that Lugus is the one who sanctions destiny that is sworn, but what about destiny that is not sworn? Could it be that destiny that is sworn is for favorable occurrences based on taboo’s like a Tynged or Geas that end in doom if those taboo’s are violated? Could it be that destiny or fate or order is passively designed until it’s not? Or could it be it was already designed and that the petitions made to Lugus are deemed worthy or unworthy, resulting in changes to one’s personal fate?
My idea is that Lugus is the one who comes with the design specifications for destiny. This coincides with the fact that he is paired with his consort Rosmertâ, who is seen as a prophetess, hearth mother, and weaving goddess in some circles. Weaving in Indo European religions is associated with magic, fate and destiny. Given that she is also seen as a seer/prophetess, it seems that she also has a very important role in this constructed system. So, if Lugus sanctions the destiny of all (personal and impersonal), then perhaps Rosmertâ is the one who actually creates the design for the destiny and how it will lie.
Yet, we also have the Matres and Matronae. As Segomâros and River Devora pointed out in their respective articles, there were many versions or rather different types of Matres/Matronae. These ‘mothers’ were gods of various tribes thought to be in charge of prosperity and of course destiny, what with their depictions of weaving and thread tools. If they are also involved, what role would they play?
If we look at them as tribal mothers, we could see them as being more invested in fate on a more personal level for the Gaulish polytheist and ‘the tribe’.
So let’s try to fit them into this system. So far, Lugus comes up with and sanctions the design for fate and destiny, brings them to his consort Rosmertâ to make the design. She then travels with Lugus to the various lands to petition the ‘mothers’ to create and weave personal destiny for the inhabitants of the land and their ancestors.
Lugus traveling to areas is based off of the idea that he is a wandering Wind Wolf type figure (based on conversations in the Gaulish Polytheism Community group on Facebook between Segomâros Widugeni and C. Lee Vermeer and ideas of Mercury being a Wind Wolf descended god in Gershenson’s Apollo the Wolf-god) , not to mention the theory that he is the Gaulish Mercury, the most venerated of the gods of Gaul.
So, the full constructed system I propose:
Lugus comes to Rosmertâ to give his designs for the destiny of all. Rosmertâ makes the design based off of the specifications of what Lugus gives. They then travel the lands to give pieces of the over all design to the mothers to create for all of us depending on which mothers we have watching us. If our destiny can be argued for or petitioned to, we are able to come to Lugus to swear a new addition or totally new design entirely. He will then judge it if it’s worthy, and then goes back to Rosmertâ, who will redesign the pattern, and then go back to the mothers with Him to create the destiny that we have sworn for.
Each action we take is the movement in the possible pattern being created by the mothers. It’s not that they control us or that we control them, but that what we do is a movement of the needle, or a spin of the thread, or the laying of the pattern. We can petition our mothers to change our design, who can also petition Lugus when he travels back, giving this system a circular feel. In short, we are intristically linked to our mothers without giving away either our or their autonomy.
Segomâros Widugeni however sees it a little differently in his version (edited a little bit from our conversation to make it a little more cohesive):
Rosmertâ has the initial vision (entirely possible given Her position as a seeress), brings it, and mead, to Lugus, who swears it, if He agrees, then She passes it to the Matres for detail work, but not before Rosmertâ weaves Fate, which is then sworn into being by Lugus, as stated previously. The Matres weave the Fate of individuals, families, districts, towns, tribes, provinces, and nations.
There you have my constructed system with a variation of it by Segomâros. As my disclaimer said, this is not attested, and is just guess work and unverified personal gnosis with linguistics. However, I will be attempting to engineer a system of destiny/fate by using Insular Celtic concepts for an arguably more fleshed out system (at the behest of Wōdgār Inguing of Sundorwīc Hearth and Lārhùs Fyrnsida fame. (Probably because he wants to swipe concepts like Tynghedau and Gessi some how 😛 )) Keep watching for it.
Sources and references
Matasović, Ranko Etymological Dictionary of Proto-Celtic, pp. 383-384
Matasović, Ranko Etymological Dictionary of Proto-Celtic, pp. 247
Enright, Michael Lady With a Mead Cup
Enright, Michael Lady With a Mead Cup
Widugeni, Segomâros http://polytheist.com/segomaros/2016/02/22/dewas-matres/
Vermeers, C. Lee https://faoladh.blogspot.com/2011/05/gods-and-goddesses-of-werewolves-wind.html?m=1
Gershenson, Daniel E. Apollo the Wolf-god