The reasonable test must be applied IMO. An IFR/TSO GPS is not required for VFR, so no problems there IMO, but can it display a CDI? I'd want to test that the applicant can track and intercept with a CDI, since that is what most of all the airplanes equipped with nav will have, in one way or another. That is also "reasonable" IMO. Also, if equipped with a GPS, select a waypoint and go "direct to". Although it sounds simple, it's amazing how many people over-rely on "nearest" functions and can not dial in an "assigned" or "desired" waypoint.
If an applicant does an awesome job of intercepting radials and airways during their XC portion, then at some point I'd fail the navs and make it completely pilotage/DR, and then if that goes well, both tasks are done. If an applicant does the most amazing job of pilotage and DR ever, but doesn't use navs at all, then at some point I'd have them do some tasks with navs, intercepting and tracking, etc. There's not "one way" to do an XC in that sense, but all the tasks must be evaluated obviously. It won't impress anyone to plan the airways through a congested area or complex airspace, nor will it impress to fly out of radar coverage and fly 10 miles from the airway when it's readily available. It just depends on what is right for the exact situation.
Many aircraft equipped with nav systems can not be flown IFR due to inspections or other equipment limitations, but they are still used on checkrides. As long as the GPS unit can display a CDI, I'd think that would suffice and the rest could be determined through oral questioning. IMO that passes the "reasonable" test, but of course sometimes reality can be different. The more you know and understand the more you can make your case as to what is reasonable.