I haven't seen too many PIFAs or IFA antennas mounted externally on metal enclosures. The occasional multi and car antenna uses this type of antenna, primarally because the PIFA can be made to cover multiple bands, eg GPS + cellular.
A PIFA antenna mounted on a metal enclosure needs to be at 90° to avoid being shorted.
If it's just for GPS, a straight patch antenna is normally more easily mounted and has the advantage of circular polarization.
Obviously I don't have any details on your application or enclosure, so I may be wrong.
If you want something really low profile antenna you could just make a slot antenna using the enclosure. Just cut a half wavelength thin slot in the lid and connect your coax across it. You will need to experiment to tune the feed point correctly. You can seal the slot with silicon or something if needed.