To answer your original question using a step-up to power a GPS will not hurt your lighter socket in any way. I use one regularly on my '38 to power my GPS. Since I've spoken with folks who run their GPS's on 6v, if and when my step-up dies, I'll run it directly on 6v.

Save a life, adopt a senior shelter pet.
The three main causes of blindness: Cataracts, Politics, Religion.
Name your dog Naked so you can walk Naked in the park.