Measuring Open Loop Gain
Open loop gain is infinite high of an op-amp by design.
See https://en.wikipedia.org/wiki/Open-loop_gain Of course, in real opamps the gain can not be infinite, but it is quite high. In real opamps the output can not go to infinite high or low, but will be clipped by the min-max of voltage rails.

In the datasheet you can see an open loop gain of 78dB, which roughly corresponds to 100,000,000 times of amplification.

However, your circuit does not seem to be open-loop. It is a driver for a closed-loop system. The transistors drive the transformer, and there is a feedback from that fed to your error amplifier, this makes this circuit a closed loop control.