I just ran into the same problem and here is what I did to work around the issue:
June 22, 2016 Google changed their policy for the API that is called in order to use the map on your site. You can read more about the steps I followed at the following site: https://www.latecnosfera.com/2016/06/google-maps-api-error-missing-keymap-error-solved.html
First you have to register for an API Key, use the following site for that, just click on the Get Key button: https://developers.google.com/maps/documentation/javascript/get-api-key#get-an-api-key
Since the WP GPX Maps plugin doesn’t provide you with a settings field for inputting your newly acquired API key, you will need to install another plugin as a work around. The new plugin is called Google API KEY, and can be found at the following link: https://ww.wp.xz.cn/plugins/api-key-for-google-maps/
I hope this helps.
