How do I compute the gain on this differential amplifier (in an open loop)?
Suggestion: these are bipolars, so you need the VEARLY (Early Voltage) in the transistor parameters.
You can examine the SPICE model for the transistor. Probably the VE param is what you need; expect a number between 10 and 100 volts.
The Vearly gives a useful approximation of the Rout of the transistor.
For the two right_hand transistors, The Rout_upper and Rout_lower, in parallel, becomes the current_to_voltage converter portion of this circuit.
The two transistors of the DiffPair are the voltage_to_current converter. Find the transconductance of that diffpair.
Then multiply the Transconductance by the Rout_upper||Rout_lower.