There is apparently something in the GA state parks verbiage that specifically mentions detectors. (and I have sneaking suspicions of the origin). Ok, fine then, you can nix state parks there.
You can do BLM and NFS , as long as you're not running afoul of ARPA (you're looking for nuggets and/or modern coins, right ?
).
And as far as down to city and county levels: In this digital day & age, there's hardly a single city or county that doesn't have their muni codes, park rules, etc.... on line. Eg.: Dogs on leash, no fireworks, etc..... If it doesn't say "no md'ing", then so-be-it. Personally, I don't hesitate to hit any park or school or beach etc... that I come to (barring obvious sensitive monuments).
But if you are skittish, you can look it up for yourself. And DON'T go asking desk-jockey's "can I metal detect ?", lest you become the latest victim of "No one cared UNTIL you asked " routine.
And of course, don't be in the middle of deep retrievals on nice manicured turf, when busy-bodies are staring. And don't go waltzing over beach blankets at an archie convention, and so forth.