Looks like a half reale, value is only the joy of the find, it won't bring much $$ because it is so worn out. I have found a few in MA myself in similar condition. I do love finding the oldies though, no matter how worn they are.
I believe the coin is not from Mexico; rather from Guatemala. The "G" you see is part of "NG" the mint mark of Guatemala. The "M" is not the mint mark of Mexico but rather the Assayer Mark for Manuel Eusebio Sanchez. The coin is 89.6% silver.
The full description would read: Obverse : Laureate, armored and draped bust, facing right; "CAROLUS·IIII·DEI·GRATIA·"; (date) below between dots. Reverse : Crowned Arms, between pillars; "HISPAN·ET IND·R·"; ·NG· at left and ·M· above left.
Don.......
Interesting, I may have to check my reales and see if there are any from countries other than Mexico. If that really is from 1797, it is a pretty rare coin. In better condition it could be worth around $200.