it's probably been 25 years since I ever remember trying to measure the GAIN of an ST70 amp. Most amps don't publish a GAIN spec.
They publish a sensitivity spec (input voltage required to produce full output power).
But if memory serves me well (not usually!) a stock ST70 amp has a gain of around 20 db and about 20db of negative feedback.
With the VTA driver board, the gain is around 30-35db and about 12-15db of negative feedback. This is all BEFORE the output stage.
Let's check around and see if anyone has tried to measure this in recent years.
Are you asking about the gain of the driver board, or the gain of the entire amp from input to speaker outputs?
In general terms, it takes about 1/2v rms to drive the amp to full output of around 40 watts.
Don't forget you are going thru an output transformer,
that couples the output of the power tubes thru a transformer of around 4500 ohms down to 8 ohms.
I know that the driver board is swinging nearly 200v p-p into the output tubes after negative feedback is applied.
So your question doesn't have a simple answer, unless you want to get very much more specific.