Yes you can. Make sure you fuse the socket to match your GPS as they draw very little amps and will not blow a 1 amp fuse ( at least my garmin 60 CX only has a 1 amp fuse>)
So that is your issue, if you want to use the socket for anything else than the gps, you will have to change out the fuse. So for example if you wanted to use a heated vest, likely in the 10 amp range or a tire pump more like a 15 amp range.
On mine I have three lines to the battery ( combined into one ground wire & two power wires ). Each one has its own fuse rated for the load it is expected to carry.
I also have a battery monitor and the GPS itself draws very little juice, so that for gas or lunch stops you don't have to shut it off, but other than that I shut the gps off.