The usual servo tester can generate a control signal for one channel. This is assumed to be the throttle signal when the pulse train signal is connected to an ESC.
There are other ways, such as using one of the Raspberry Pi micro computers and a "servo hat", and/or a "Hat that has both servo control outputs and motor drive outputs. I'm not sure what you mean by gain, since that is usually a function of a gyro.
The ESC governor mode is intended for heli use, and sets a desired rpm. Other than throttle control, most ESC settings are made via programming before use, not during it.