^^ You could hear it in variety of situations. So it's so hard to give you an exact explanation.
우리 (먼저) 갈게요 - (When a couple of people part with their friends, they say that.)
우리 갈건대? (너는 어떡할래?) - (When a couple of people part with their friends or they are supposed to go to another place, they say that to know what plan they have.(leave together , stay here or etc,.?) )