My hot tub is rated 40 amps what size gfci should be used? The same size or larger?

You should go up one size on your breaker to a 50. It will break too easily if you size it at 40, due to in-rush amps when you first turn it on. Not many are installed with a gfci breaker, but in some states it is probably code. You may have a hard time finding one in consumer based stores.

I am in the water treatment business and do work on commercial pools and spas also. We don't use GFCI on commercial stuff here in FL, except for outdoor electrical outlets and stuff. You may actually find an electrician if you re-post in electrical who can tell you exactly.

Make sure you are using the correct size wiring to handle the load. Without looking it up, I think you probably need a #8awg wire depending on the distance you are running it. It is best to run the wire out to a service disconnect or single breaker panel at the tub, and then a short distance from the disconnect/breaker to the tub. That way if you go to work on it, you don't have to go to the house to operate the breaker.

Al