Musar zu Bamidbar 32:2

As to the reason why Gad is singled out here and no mention is made of the other one and a half tribes who were a party to the same agreement, I think it may well be that since the Torah had reported the tribe of Gad as the first to demand its share of the land on the East bank, this tribe had been more suspect than its partners as perhaps unwilling to share the problems of the other tribes. When even this tribe assumed a greater burden than had been required, the Torah rewarded it by mentioning it first always.