GoAir has been shuffling terminals at Mumbai over the years. Earlier, they used to operate from Terminal 1B since their initiation of operations. About a couple of years ago, after the demise of Kingfisher Airlines, they moved their operations to Terminal 1A, and co-habited the building with Air India between 2013-2015.
However, yesterday at the airport I noticed a new change which is being implemented as you read this. I checked the website and also found the same update.
So, basically, all the flights departing from Mumbai will depart from Terminal 1B, which is the same terminal where Jet Airways, Indigo and SpiceJet operate from. They have been offered 7 check-in counters to operate with at the moment. However, on the way back, passengers will arrive at Terminal 1A, which is the terminal building which Air India uses (and GoAir used to use till yesterday for both arrivals and departures).
Confusing much, but one to take note of if you are flying GoAir.