Did some testing: char St 13, not at potential, wore GoGS +12 St (so effectively at 25), paid Garth 2000 gp, waited a lot, nothing. I think increase should have happened.

Same char, St 13, not wearing GoGS, paid 2000 gp, got St increase as expected.

(tested in p20, did not see any changes about Garth since then, sorry if not valid anymore)