Or HaChaim on Numbers 32:31/אור החיים על במדבר ל״ב:ל״א

1-32. ויענו בני גד, The members of Gad etc. replied: The Torah had to repeat matters once more and also could not simply write: ויאמרו, "they said," but had to write: "they replied." Seeing that Moses had said ונתתם, the two and a half tribes had begun to fear that such a handing over would only commence at the end of the fourteen years of their whole-hearted participation. This is why they described their forthcoming participation as so firmly anchored in an unbreakable agreement that although their part of the fulfilment of the agreement was some time in the future, the rewards for abiding by it accrued to them as an immediate benefit, i.e. the land was to be given to them as of now. This is why they said: ואתנו נחלתנו, "the possession of our inheritance will remain with us." They had not properly understood Moses' words, as we have explained them. As a result, Moses gave them the land immediately to prevent any misunderstandings.

2It is also possible that initially even Moses had not meant for the two and a half tribes to actually acquire their claim until after they had demonstrated fulfilment of their part of the bargain. However, once Moses realised how firmly these two and a half tribes had committed themselves to helping the other tribes he agreed to give them possession of these lands immediately.