If i were a home build sort of guy, i would get a VCO in the right frequency range, and use a potentiometer on the tuning port to manually sweep the output frequency.
I would use an Analog Devices LOG detector, with an attenuator pad in front of it, to measure the VCO output power, and then the amplifier output power as the VCO is swept in frequency, and deduce the gain from that.
https://www.digikey.com/product-detail/en/analog-devices-inc/AD8362-EVALZ/AD8362-EVALZ-ND/1543401