I'm using EE 2.8.1 and Gmap 3.2.7. I'm suddenly getting a message "You must enter a location in the "custom field" field" when I try and save an entry. This is happening even though we have entered a location;
There seems to have been a recent change in the gmap API. See if this thread can shed some light on your issue.
gmap/ft.gmap.php might help.