can not get required gain with gilbert cell on ltspice
You "blindly" connected square wave sources (V3, V4) to the inputs.

The problem is that you do not define a proper commonmode level. I would try to make the commonmode voltage of V3 something around +10 V and for V3 use +5 V.

ALWAYS first make the DC solution in order (check the biasing of each transistor) before applying signals to it. Applying proper biasing is essential and a step that you missed!

Here's how I make a differential signal with a properly defined commonmode level:







simulate this circuit &ndash; Schematic created using CircuitLab

Vcmm sets the commonmode voltage

Vin is the input voltage, note that if I want a 1 Vpeak-peak differential signal that I need to give Vin an amplitude of 0.5 V (not 1 V)