Fol. should be of interest to get you started re. Low Power FM TX in CANADA (ONLY):
Max power is usually stipulated as Effective Radiated Power (ERP), Relative to a Dipole....which is Max allowed to be emitted by the Antenna.
If using [the popular] Vertical Dipole [TRUE OMNI], Max Antenna Gain is 0 dBd (= 2.15 dBi), so ERP is same as Transmitter Output (less hopefully small Coax Loss).
OTOH, if you chose to use some Other, Higher Gain Antenna, such as for example a "typical" FM Turnstile Antenna, Max Antenna Gain could be HIGHER , since most are Bi-Directional and NOT OMNI, so must use corresponding lower Transmitter Power: