GFCI Trips Single Pole Switch
Why are you using the LOAD terminals?  You have no idea what they do, but what you do know is the GFCI is a different type of thing and the screws are even differently labeled.  I am just mentioning this because mains power can get people hurt or burn your house down.  "Not knowing" should make you more reluctant to experiment.  

Pull the GFCI out of there, cover both the LOAD terminals with tape, and don't use them.   Now your project has a chance of success.  

That will create another problem to solve, and the answer is "pigtail".