I believe the answer is only after the rest of the land was conquered but before the land was allocated, and all this happened after the death of Moshe.
See Bamidbar 32:29 where Moshe instructs Eleazar and Joshua to give the land
Moses said to them [Eleazar and Joshua], “If every shock-fighter among the Gadites and the
Reubenites crosses the Jordan with you to do battle, at the instance
of the LORD, and the land is subdued before you, you shall give them
the land of Gilead as a holding. But if they do not cross over with
you as shock-troops, they shall receive holdings among you in the land
Artscroll quotes Abarbanel that, although Gad and Reuven were ready to stay in Israel "mainland" until it was allocated to individual Jews, Moshe declined the offer and said that they need remain only until the wars were over (which took seven years) but before the other seven years for the allocation of land).
From the language in Joshua 13:8 and following (e.g., 13:15), it appears that Moshe assigned the land that was promised to them, it was then occupied by women and children but it became their formal possession only after the wars. Indeed I have now seen that artscroll on these verses quotes Lev Aharon: "the tribes on the east bank did not acquire full ownership of their territories until Eretz Israel proper was in Jewish hands. See also Joshua 22.
On the question raised in comments of whether these territories were part of Eretz Israel, the gemara in Arachin 32b mentions that when the tribe of Reuven, Gad and half of Menasheh was exiled, the Yovels were suspended (since they apply only wen every one of the 12 tribes is settled in Eretz Israel in their respective places) which I think proves these tribe territories were in Eretz Israel (see here for further discussion).