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.
Apparently changing
$this->EE->theme_loader->javascript('https://maps.google.com/maps/api/js?sensor=true');
to
$this->EE->theme_loader->javascript('https://maps.google.com/maps/api/js?v=3.22&sensor=true');
in gmap/ft.gmap.php
might help.