GpsGate Server v2.3.1 used Google older v2 geocoding API. GpsGate Server v3.0 uses the newer Google v3 API for geocoding.
Google says it will at some point disable the older v2 API. But it seems to work for now, and also allow a higher daily quota than the v3, which allows 2500 requests per day for a free key.
Here is a Plugin which enables usage of Google v2 geocoding API in GpsGate Server v3.0