Saving GPS Coordinates in Contacts
According to Apple's developer documentation about their MapLinks  iOS/macOS supports map urls via a web link (using http or https) to maps.apple.com - so to link to GPS coordinates one uses:

https://maps.apple.com/?ll=50.894967,4.341626


Also just to add it seems that 'geo:' maps scheme standardised in RFC5870 sadly doesn't appear to work on iOS/macOS. Although according to the geouri.org there is some support in Android, and other applications.