I assume that you mean the GPS receiver side of the unit (rather than
the FRS/GMRS 2-way radio). I'd guess that the performance would leave
something to be desired. But then that is
dependent on the type of airplane. I fly a Cessna high wing, and would
never be witthout an external antenna for my GPS. No doubt the built-
in antenna would work fine under a bubble
canopy. While on the subject, I'd also comment that trying to make a
non-aviation GPS serve any useful purpose in the cockpit is difficult.
Certainly it is possible to do so - but it's a lot
of work.

Dave