GPS receivers typically only use their own mapping and not link to other devices to input information.
To use a GPS in conjunction with the app, all you need to do is input the address of the campsite into the GPS.
If the campsite is in a remote location (eg: off road, not shown on normal road maps) then you may need a GPS with off-road maps.
Most GPS brands offer these options and there are some that are dedicated off-road devices.
As for what the app can and can't do in regard to interfacing.... why not ask in chat support on that app's website?
As for buying a second-hand GPS.... many older models don't have free map updates.
Sure, there are ways to get these from other sources, but with prices quite reasonable for new GPS models with free lifetime maps... why bother?
Garmin, TomTom, Navman... all do the same thing.... it's really just a personal preference.
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