Don't know when your trip is and thus whether you can duplicate what you have through a reputable channel. But, I would at least price this ticket through EI and if it can be had to anything close to what you had, I would book the new ticket and cancel the Gotogate ticket (or handle through a chargeback given the likely answer).
A quick scan of online reviews suggests that you are not alone, that the vendor won't help you and, at best, you are stuck with what you have and at worst, wind up with something much worse. Better to get out while the going is decent.