Letting them know before indoc is still a good idea, but the other half of that equation is that there have to have been unfilled vacncies in GUM to give it out. So, the last several vacancy bids have not had any unfilled assignments to GUM, so even if you want one, they can't give it to you. If GUM is your ultimate goal, you would definitely want 737 out of new hire, so you could just lateral from some other base as soon as your seniority could hold GUM.

Vacancy bids are coming out every 4-6 weeks, so depending on when you are starting, you will get a chance to bid to GUM very quickly or if it goes unfilled before you start you could potentially get it straight away.