I am going to give you general answers and not specific sites. I metal detect and want you to know that I have never found a gold coin.
Since it has been awhile since gold coins were in circulation, you can imagine that the places where gold coins might be buried would be old buildings, the oldest parts of towns and cities, and places where there were settlements (including ghost towns).
Now, you have to be careful, because some places are on private property and you must ask permission to detect. Also, I believe you can metal detect in National Forests near picnic tables and camping areas. However, check this out. Any other public property is off limits except recreational areas where gold panning is permitted. Again, check it out.
Finding an abandoned town or village that isn't protected would be your best bet. Next, in my opinion are places where people sit. Coins have a habit of falling out of pockets, that is why picnic table areas are such favorite spots. I like to metal detect under shade trees near a stream, creek, or river. People have picnics, sit to fish, etc and I have found coins in those areas. My best find was an Indian Head penny. Everything else has been modern coins and mostly pennies. Another place, if you could get permission would be a fairgrounds or under bleachers at ball parks. Of course, if the area was not a sports area in the 19th or earlier twentieth century chances are you won't find a gold coin.
I know people who have metal detected in single abandoned old homes and have found coins below floorboards. They also find a lot of other stuff. A friend of mine found a coffee can in great shape from the 19th century near a gold claim. Maybe some old prospector made it his shack.?
There are some tips. I'd find a shade tree!