Samsung S4 GPE - "GPS Signal Lost" Dilemma. Considering Factory Reset. Need Backup advice
Had same problem but was able to solve by:


open back cover
take a very small phillips-head screwdriver
remove the battery, SIM Card, and Data card
tighten all screws, especially the one on the top left hand when the back is open. Reason:  Behind that screw is the GPS contact point touching the GPS antenna, and this gets loose very often.


For me this solved the problem.

Download GPS Status and Tools from Playstore and test.

Works fine now.

Even Samsung couldn't solve this over phone, but asked me to leave the phone with their service centre which will take a week to fix.