Same issue here with iPhone 7+ 128gb matte black. I am a driver and depend heavily on the GPS, I use it about 15 seperate times a day. I never once for nearly 2 years had a problem with my iPhone 6 GPS. From day one with the 7+ it's been a huge headache. It only works about 50% of the time, the other half of the time it gets stuck in a field, or doesn't know where I'm at. The problem usually last for about 5 minutes before it finally figures out where it's at again for a few minutes.
I had an idea to open up the compass app while the phone is lost, and the compass had no clue where North is.
So I got sick of pulling over and waiting for the phone to set course again while I'm in unfamiliar neighborhoods, and I made an appointment with apple.
The first thing they tried was resetting the phone as new without an iCloud restore. Needless to say this did not help. They said bring it back if I still had an issue and they would probably replace the phone itself.
So about 4 days later I was finally able to get in with an appointment to an apple store, and the extremely nice guy replaced the phone. I thought for sure this nightmare would be behind me.
The replacement phone is just as bad as the first one, the GPS is completely useless.
So now I'm trying to decide if I want to downgrade iPhones and go back to a 6, switch over to Android, Or shell out some money for a Garmin.
At this point I am furious that I have an $850 telephone that can't find itself. I don't know where to go from here.