One tip I can give you that I always see my GSAs stressing out about is to also learn how to let things roll off your back. There will be times where ETLs don't give your team a lot of hours to work with. So while they're enjoying saving hours and their bonuses, you're stuck with the stress of having to plan breaks/lunches with no coverage and guests yelling for more lines when there's nothing you can do about it.
That being said, your other GSAs will become your best friends because they know that stress and have felt it, so they will definitely help you cope.
One thing I can say is that it takes a special kind of person to be a GSA in the sense that you tend to feel more stress than you need to feel for a job you'd think would be really easy.
As for me, I'm glad I've declined my ETL's offer of becoming one mainly because I don't think the stress and time is worth the 50 cent raise especially while I'm going to school.
I don't mean to scare you, I just want you to know that it'll be difficult but it is a good stepping stone for another position. I wish you the best of luck though.