Datasapiens but if he installed all in apk and does't have the google in the phone at all?
Based on the following:
oidfjgxccawqe I installed Signal via direct APK, but then was prompted to install and allow access to Google Play Services
It seems like he indeed has GPS installed, even if some apks comes from different sources.
I guess what Johnnyloans is suggesting is a very good starting point, and step 2 should reveal most of the apps that require GPS.
If there are no corner cases, this might be enough to have what the OP is seeking.
oidfjgxccawqe and check if functionality breaks...
You get the point, and even if you installed Signal from a different source, stopping Google Play Services might break notifications.
Keep in mind that some apps have a fallback solution if they don't find GPS, so they may continue to work without it (Like in fact Signal).