I don't see anything in the GITkit documentation where Google promises any kind of uptime number or notification window before disabling the service and leaving

I don't see anything in the GITkit documentation where Google promises any kind of uptime number or notification window before disabling the service and leaving you with no way† for your users to authenticate to your service. † You could try get them to do a password recovery procedure since you kept their email address, but I'd rather not send out an "I'm sorry, we lost your password." message to customers.