There are many adapters that can be used and many have been posted here–try a search using the search box above.
The short answer: as a general matter, the Clip can use a “basic” standard USB car/AC adapter, connecting using the Clip’s supplied USB cable or where the adapter’s permanently attached cable will fit the Clip’s standard mini-USB port. Examples include Motorola Razr adapters; Mio GPS 310x car/AC adapters (personal experience); the Macally AC adapter labeled for Sansa players (even though the Clip is not listed) (personal experience); the DLO Power Bug AC adapter listed for general MP3 player use (personal experience); and many others. Although the adapter must have sufficient amperage for the Clip, as the Clip requires a lower amperage, that has not seemed to cause issue.
If your GPS adapter supplies standard USB power and has the proper connectors, it should be fine–the USB standard is precisely that, a standard. But I can’t guarantee this or the above–you never know when a manufacturer might have done something different or where an adapter is off. But this hopefully will give you an idea of others’ successes.