My headcanon image of Gearlt would reject hatred toward trans people to degrees depending on the situation:

Trans person being directly discriminated against or even physically assaulted: would step in to either threaten the instigators off or drive them off with violence.

Bigot is making offensive comments: would get a patented Geralt "hmm" and along with a mental note of how this person is a bigot, possibly a glib remark at their expense, or just calling them out for their shittiness

Overall, I view Geralt as being pro-Trans being canon in the sense that he doesn't really stand for discrimination or hatred toward any minority.

But if I'm being intellectually honest, his specific views on the topic would be up for interpretation, as we haven't seen actual trans representation in the series.