GraphQL by PoP will soon be available as a WordPress plugin! Yay!
I'm currently working on it. All functionality is already working, which is the most important stuff. Only the tasks belonging to packaging the plugin have to be done. Then, I believe that it should be ready within a couple of weeks, and then another week to have it published on the WordPress directory.
Until then, you can follow the instructions here to install GraphQL by PoP in WordPress, through Composer.