By agreement in gender and number—but not case, since the pronoun's case in the subordinate clause may differ from that of its antecedent in the parent clause.
By sense—that is, by what interpretation makes sense in context.
You're running into trouble, though, because qui in that sentence has no antecedent. It refers to the as yet unnamed inhabitants of the third part of Gaul. It might help to see another sentence that works this way:
Qui desiderat pacem, praeparet bellum. (Vegetius, Epitoma Rei Militaris, III)
Who desires peace should prepare for war.
English has this construct, too, though today it sounds old-fashioned to use "who" in this way, without being preceded by another pronoun, like "one", to serve as an antecedent.
The pronoun together with its subordinate clause serves as a noun in the parent clause. The same principle is at work in the phrase "To whom it may concern". The whole clause "whom it may concern" is the object of "to".
Here are another couple examples from the same book by Vegetius (ch. 25, on what to do if you've been partially or wholly defeated):
Innumerabilibus hoc accidit bellis, et pro superioribus sunt habiti qui minime desperarunt. Nam in simili condicione fortior creditur quem aduersa non frangunt.
This [a comeback after suffering losses] has happened in innumerable wars, and [those] who least despaired were found victorious. For in like conditions [of partial defeat], whom adversities don't break is believed stronger.
Here's an example well known in both Latin and English:
Quem Iuppiter vult perdere, dementat prius.
Whom Jupiter wishes to destroy he first makes mad.
I've seen this construct often used to introduce new terminology, as in your sentence.