I have the same issue. After upgrading iphone6 to 9.3.4 last week, the GPS stopping working. It keeps triangulating but cannot provide directions or show where I am, while driving.
Took an appointment with Genius Bar and went there today ( Aug 13th 2016 ). They restored the OS 9.3.4 again and asked me to drive around and test it. Still the problem exists.
Went back to them and asked them to restore my software back to 9.3.1 version which used to work. They refused to do it.
Now they are asking me to pay $299 for a flat repair cost to replace my iPhone 6. Asked them what happens if it fails again after the 90 day warranty window. As usual pay another $299 or buy apple care warranty.
They refused to acknowledge any issue with iPhone 6. Even after me referring to multiple threads on apple support community, they didn't bother to hear me at all :-(
How do me make Apple hear us out and do something for their issue. It definitely seems to be a batch/lot issue with certain iPhones.