I am aware that GFCI circuit breakers exist, but when installing an
additional 15 ampere branch circuit, an electrician installed a GFCI
device within an one gang electrical box. How does such a GFCI device
compare to use of a GFCI circuit breaker ? Cost? Reliability? False
alarms ?

How might I go about testing the newly installed GFCI device ?

The additional GFCI branch circuit is required for running a Panasonic
exhaust fan within the wet area.

If I had known of such independent GFCI devices, I would have made use
of it rather than individual GFCI duplex outlets when I added a
circuit branch for auxillary outdoor and garage duplex outlets.