I'm not surprised really--it's hard enough to find carriers which will set you up with a Treo, let alone other licensee's Palm-based smartphone. I wound up with a Sony just because was small and had bluetooth. It happens to have a not dismally useless camera as well.
To try and answer the question, I don't think the GPS (which isn't really GPS in this case) is accessible to the user in any way that a mapping program could make use of it. Phone vendors go out of their way to prevent you from knowing that your phone is capible of reporting your location to whomever you happen to be speaking to, even if that feature isn't enabled by default except for 911 calls.
The GPS on the 7135 is actually aGPS (assited) and at this point only serves to inform 911 or Verizon of your location. It is conceivable that the tech could be used by the phone for mapping but the computing of the actual location is done by Verizons servers and you would have to have an active signal connecting your phone to them for a constantly updated location placement, and I doubt you'll see that kind of app developed for the aging 7135.
You should try pdaphonehome.com it has a rather large and active 7135 forum. I got a lot of info there when I had my 7135. Over 40,000 posts!