Greg wants to know if he uses a magnet to connect his phone to his dashboard, will it affect his GPS? Leo says no, it won't. In fact, many cases, like Rokform's, can use a magnet to enable you to do just that and Leo has not seen any issues. It could screw up the compass, if he's relying on that, but it shouldn't affect his GPS.
Greg has been having issues with his map apps, though. Leo says that GPS does rely on line of sight to the satellite, and as such, connection to your GPS may come and go depending on obstructions. Apple also loves to use metal in their case design and that can cause interference which is why Apple has to design places for the signal to get in and out. So if he were to block that section, it's possible to have GPS issues. Changing the case he uses might solve the issue.