I'm assuming that you're talking about an adapter made for the laptop, as opposed to something like a power inverter. It should be fine as long as you have fuse protection, of course brand name matters here. You don't really want some generic adapter that may not be a perfect match for your hardware. As far as the safety goes it should be fine. Just keep the engine running while you use it so you don't wear the battery down.
If, on the other hand, you're talking about a power inverter you need to look at more than just the fuse protection. Namely the wattage supplied by the device and the voltage regulation.
I've done both for more than 12 hours at a time before and I don't see any problem with it. I did the same thing until I broke down and installed a mini-itx computer in my car. Now I've got GPS, OBD II diagnostics, mp3 and video. With a touchscreen too
BTW: you can always buy the GPS in the first link and try out the demo of the software, then just buy the full version if you like it.