It is easy to prove that Differential Gain=2*(Single-Ended Gain).Speaking in dB : Differential Gain=Single-Ended Gain + 6dB.
So,both results are correct.
Since the equation and schematic are taken from the same literature it corresponds to the Differential Gain (gain1 as you name it).
But to be absolutely sure put the values that simulator gives at it's results on your equation and evaluate it.You will get the answer for which case it is written ;-)