Its a simple engine called FPS creator. Its not a hard engine to use, its pretty simple and fun to be honest.
Hell yeah! And also just noticed that it is open source but here you go anyways: https://github.com/TheGameCreators/FPS-Creator-Classic
On that page there's 2 links. 1 says FPS Creator Classic and the other says FPS Creator Classic Update V120. Which did you use?