It depends on which one you buy.
All of them will still tell you where you are anywhere in the world, but the map coverage may be limited to a particular region.
For example I use a Garmin GPS90, this has no maps as such, but does have a Jeppesen database for all the airports, Navaids and CAS. The Jepp database can be updated to cover any region of the world, but only one region at a time (think the choice is something like Americas v Atlantic v Pacific).
I think the cleverer (?!) ones have a built in basemap covering one region only and this can't be changed but you can load other maps on top of that giving more detail but with smaller area coverage - this can be either in the basemap area or elsewhere. This bit is certainly true for the roadgoing GPSs, not 100% sure if this is also the case on the aviation ones.