In Vezos Habracha where Rashi says on Perek 33:18 that the last 5 tribes he doubled over, Moshe did this because these tribes were weak so he wanted to strengthen them. However Rashi later on, on Perek 33:21 says on these words, being, ראשי עם, Rashi says that the reason why they would go ahead first is because they were mighty. So were Gad strong or weak?
See Rashi to Bereishit 47:2, who notes that this is a matter of debate between Bereishit Rabbah, which believes Gad was strong, and Talmud Bavli and Sifrei which believe that Gad was weak.
Rashi to Devarim 33:18 is paraphrasing Sifrei. Rashi to Devarim 33:21 seems to be following the opinion of Bereishit Rabbah.