In floor heating GFCI thermostat failure?
You cannot always interchange the thermostats in this application. As long as your thermostat is a transformer coupled switch (essentially closes the contacts between two wires to call for heat) you can. If it uses live AC voltage, or serial communications, you have to replace it with another suitable replacement.

You will have to identify the make/model of the thermostat, and verify compatibility with what you would like to buy to replace it. Now with that out of the way -

Since you have a zone that is not failing, you can swap the GFCI &amp; Thermostat from that zone to your most suspect zone. This will QUICKLY isolate if the fault is in the gfci or thermostat.

My guess, it is not.

More than likely, you have breakdown in the heating coils, and via environmental moisture (for example), the circuit is leaking power to another path, which is very dangerous both in shock, and fire concerns. This is why you have a GFCI. Given that nuisance trips DO HAPPEN, it is understandable that you'd reset it, but given that it has been 15 years, and it is happening more frequently, your system is likely telling you it is failing.

Get a heat camera (FLIR), and isolate hot spot(s).... once you identify them... and there may be more than one in one circuit, you can decide to repair or replace.