Referring to Voltage Gain vs. Frequency (pg. 6), note the prominent slope where gain is dropping with frequency over six or seven decades (it makes a very good integrator!). This slope is guaranteed by design (as is the parameter itself..), so all that matters is measuring the gain at a particular frequency in that range. From that, you know all points on that line, including where it intersects unity (G = 1).
Damn, that's a nice amp too, I'd pay ten bucks if I needed one. And they aren't even that, more like five bucks.
Tim