Premarital counselor gift?

Nope, not a thing at all.

I'm guessing if you are asking this question it is probably because the counselor has made a positive impact? If that's the case and the time you and your FH spent with the counselor was valuable, then a gift shows appreciation, but as the others said, not a thing nor is it necessary. Just do whatever feels right to you.

Is your counselor your pastor or someone who personally knows you both? Or are they providing their services free of charge? These are the few circumstances where I'd get them a gift to show appreciation. Otherwise, I don't think this is a thing.

I think if your counselor left such an impact, a sweet note would suffice to say so.

I would not invite my counselor to the wedding unless you have a personal relationship with them beyond therapy

If you are already paying for the service, I think that's enough.

I mean, if you really feel like you want to do something for them, you can but it's not a "thing."