Can I run a heated floor thermostat on a 30 amp sub panel with a 20 amp GFCI breaker?
There are lots of variables here which need to be fixed to give a definitive answer, i.e. size of heated area in bathroom, 120 or 240 V heating cables, other circuits/loads on the 30 A subpanel.

However, most thermostats intended for electric in floor heating already include a GFCI, so you need to check your thermostat specs before installation, because a GFCI breaker would not be necessary or recommended in this case. However, a dedicated breaker (GFCI or not) is recommended for the floor heating (and required in some jurisdictions).

Assuming your bath is typical and not Caracalla-sized, then it would work out to between 30 and 50 sq ft heated area (you don't heat under the shower, toilet or sink/vanity), and at 12-15 W/sq ft (depending on the heating system) you're looking at between 360 and 750 W for the floor heating. This represents a max load of 7A @ 120V, or 3.5A at 240V, so not a huge load on the subpanel (depending on your other existing circuits/loads). Certainly a 20A breaker for the floor heating would be adequate. 

If the heated area of your floor exceeds about 120 sq ft, then you could exceed the capacity of a 20A breaker. Above about 50 sq ft of heated area, I would recommend a 240V in-floor heating system in any case to balance the load on your 30A panel.