Look it up in your redbook. You've been posting on here for awhile and have read that it's a great book for new collectors. Surely you bought a copy by now.
Check your Red Book or at least USACOINBOOK (and buy a Redbook .. it is the *first* thing you should have before, before starting to collect coins). if you look at usacoinbook .. first the 1999 S ... there are no G-MS grades. Only PR - Proof grade. So they only made a Proof coin. Keep in Mind that San Francisco is known for making proofs (check your redbook). yours is an impaired proof - impaired meaning damaged, put in circulation, etc. there's also many threads here on CT that review the physical differences between Proof and Circulations coins or just generally google it.