For the rare instance, Geralt laughed. Seeing Jaskier speechless was pretty humorous. But no matter how funny it might be, he wasn't trying to simply mess with the bard to be cruel.
He canted his head as he pointed out aloud, "I imagine this is how others feel when they interact with you in this manner." Jaskier was flirtatious and got around. It was no secret. The tables being shifted was interesting to Geralt, especially when the other could be so...oddly cute.