dBi is the Gain (in dB) of you antenna related to that an isotropic radiator (which.... doesn't exist in practice).
I believe you can simply consider the Gain in dB you get in HFSS as if it were in dBi.
On the other hand:
- dBm and dBW are relative to Power (not Gain)
- dBc stands for decibels below the carrier (it is used to define the sideband noise properties of oscillators)
For plotting the gain over frequency you need to make a frequency sweep and activate the "save field" option. Then you select under plot Far Fields Reports -> Rectangular plot. Now you can plot the gain versus the frequency.