Although it doesn't happen often, in rare situations you might get in trouble with GMAP marker sets. The most common case is change with folder permissions or migrating site to another host.

How to add Custom Markers for Gmap Module Drupal

Here is how it looks:

Request for invalid marker set notification

Request for invalid marker set notification

And here is how map field looks:

Map with missing marker (s)

Map with a missing marker (s)

The default map is displayed.

Luckily there is a simple solution - delete gmap.markers.js file from files/js folder, and markers will regenerate by themselves.

sudo rm -rf  gmap_markers.js

Now, run clear cache and all markers should be back

Marker cache cleared

Marker cache cleared