What was Moshe accusing Gad and Reuven of?
Rashi: He was accusing them of putting fear into the hearts of the other tribes, 1 who would think that they were afraid of crossing the Yarden to fight the Cana'anim.
Ramban (in Pasuk 2): He was accusing them of a lack of Bitachon like their fathers, 2 as a result of which they were afraid of the Cana'anim and did not want to cross the Yarden. 3
Seforno (in Pasuk 6): He asked them whether they really thought that their brothers would agree to cross the Yarden to conquer the land whilst they remain in Eiver h'Yarden - knowing full-well from the previous generation how those who turned others away were punished.
Even though they themselves were not necessarily afraid.
Ramban: As the Meraglim said in Sl'ach L'cha, 13:31 "Lo Nuchal La'alos el ha'Am, ki Chazak hu Mimenu!"
Ramban: And this in turn, would result in their having to remain in the desert a second time (As implied in Pasuk 15 - See Targum Onkelos and Targum Yonasan there).
