never played with both, but from general principals . . .
GPS L1 is at 1.5G -- second harmonic is the closest overlap, but still fairly clear of 5.8G (and vice-versa). you probably won't use L2-4, but the same applies. The internal filtering of the GPS antenna should remove signals from the loud transmitter, but it could affect general sensitivity. You might not need it, but in general I'd recommend get out of the near field of the transmit antenna and you'll be fine -- 2 wavelengths should be more than enough.
To find wavelength (assuming you don't know this already -- if so, others might not):
L (in meters) = 300/F (in MHz)
so 300/5800MHz = 0.052m, so I'd recommend twice that, or at least 10cm separation.
The positive side of this, the GPS antenna's mainlobe should be mostly upward, while the CP antenna will have a null there. there's still a lot of overlap, but again, between filtering and about a hand-span's worth of separation, they should play well together.
hey Dan, thanks for the wavelength stuff, dont wanna derail the conversation, but just wondering, is there any good resources on stuff we can read up on or watch in regards to wavelengths, 2nd harmonics and all that jazz?
Good is a relitive term -- most of this I've gleaned from some pretty ugly college courses, but I'll poke around for a frequency basics tutorial. Probably something useful to have around here.
The concepts themselves are wierd but reachable, however finding a source that only hits the high points w/o the deep dark magic is a little more difficult. Dunno when I'd have a recomendation, but I'll be digging around the HAM sites/tutorials -- if you want to start searching yourself, it's where you'd find it.