I think this is a position that depends a lot on your store and (especially) your leadership. I am a former GSA/SEA (Service and Engagement Advocate; the title changed with modernization). It can be a stepping stone to a promotion. (I know of three people who were promoted; one eventually made it to ETL. It isn't a guarantee, though, so beware carrot-dangling.)
Things about the job I think are universally true: GSA is a limbo zone where you're seen as a "manager" by guests (who don't know or care about Target's structure), but you really aren't. The majority of your guest interactions become negative as a result. As GSA, you can't coach, etc., and TMs know it. Some (not all) front end TMs will abuse that knowledge when you're in charge of the front and no other TLs/ETLs are around. You have to be able to stand your ground, and not be afraid to report them to people who
can document and coach if necessary. You have to be able to deal with all of that and more as TL, so if you have your sights set on that, it is a good stepping stone/training ground.
As GSA, I had an hours guarantee, but I think that's ASANTS. I was supposed to have every other weekend off. (Again, ASANTS.) Because people rarely stayed in the position, that didn't happen often. When we were fully staffed, though, HR always honored that guarantee. I'm sure how GSAs are treated by leadership is another case of ASANTS. In my case, my GSTLs at my old store (and new one, for that matter) were great. The fantastic GE who hired me was, unfortunately, replaced by one who was impossible. The three-way pressure from said impossible GE, unhappy guests, and overworked cashiers/GSTMs (who had my sympathy, but who I could do little to help) was too much for me. Losing my pay differential (as
@Amanda Cantwell mentioned) felt like a slap in the face.
I quit Spot to pursue another opportunity, and when that didn't work out, I switched stores to dodge the position (and, frankly, that GE). I've had a lot of company. The GSTLs remain, but only one GSA from my tenure is still there. (GSA tends to be a high-turnover position.) The others have transferred to other areas/stores -- not always as TLs -- or left altogether. I'm happy to run the front to cover 15s/meals at my new store, and to otherwise support the GSTLs/GSAs. I don't want the job again, though.
Search "GSA" here and see what others' perspectives on the position were; don't just take my word for it. As with everything else, ASANTS rules. Consider your own store and most important, look at what you want and where you want to go. Then make the best decision for yourself. Good luck!