When the address in the model is changed, the map location isn't updated automatically - this is known and I found a solution on stackexchange.
But I wonder if it wouldn't be possible to add to the gem itself a solution like this: http://stackoverflow.com/a/10574422/855874
I wouldn't add extra complexity to the gem for this purpose.
Concerning your solution, I don't like the idea to duplicate data in the db. You'd rather do something like (untested):
self.latest_address = self.gmaps4rails_address
self.gmaps = false if self.latest_address != self.gmaps4rails_address