Re: Mileage discrepancy
GPS will not be as accurate as the car odo. GPS is radio based so reception, tree cover, bridges and other structures will impact it. Additionally will not react as fast to speed changes. Those small things will accumulate over time. 5% seems like is within error range for GPS.
I wouldn't worry about it. Continue driving it and you will see the gap widen as well.
2014 Jeep GC, V6, Black/Indigo with Adv Tech Package and ORAII