Do you think GG still would want to be friends with Taylor Swift if TS quit the music industry tomorrow and enrolled in college? I think GG still would.
And if the tables were turned, I think that TS still would want to be friends with GG.
I am not a Taylor Swift fan, but you are missing the point that a household name like TS has absolutely nothing to gain in terms of self-promotion/glamour/etc. from her friendship with GG.
Sheer friendship (priceless :yes

is the "only" thing TS has to gain from her friendship with GG.
GG is someone with whom TS feels comfortable baking cookies and hanging out, etc.

Suspecting that TS has an ulterior motive here just makes zero sense.