First, get your output to single ended using an A=1 vcvs.
Then, get your input stimulus set up so it's also differential
driven (@ legit input ommon mode) from a single ended
primary source.
You can then feed that node through a feedback network
from output to input that puts the amp in its "happy place"
for a DC result, or sweep it and use calculator cross()
function (with some somewhat elaborate args, which you
would have the fun of building up). Personally I like a
clean DC test loop with a gain block and attenuators, old
school (but needful of accommodating the fully differentlal
style).