what other gemaras quote the Reuven-Gad conditions?
The main source is Kidushin 61a-62a, and it is also discussed in Nedarim 11a and 13b, Shevuos 36a, Gitin 46b, Yevamos 53a.
All the best,
Reuven Weiner
Can you explain it in context?
Moshe makes the conditional agreement with Reuven-Gad and the Gemara learns from here that this is only if the condition was said in a certain way, then the stipulation is Torah-legal, but if not done in the right way the stipulation is not honored and the act will take effect, ignoring the conditions he said (the act was agreed on, the condition erased, so the act remains).
See Shulchan Aruch, Even ha'Ezer 38:2,4 for the rules in a condition in Kidushin.
In short, there are 5 points (4 in the Mechaber, plus 1 in the Rema): mentioning both sides, positive before negative, mentioning the condition before mentioning the act, condition must be possible, and the condition "not in the same point as the act" (see there).
All the best,
Reuven Weiner