Hey Greeneman42. Good question! At the bottom of the rules it says:
* You may assemble your team and discuss possible ideas. But avoid designing and DEFINITELY do not create any game specific code before the start date. ( Plugins, engines, and frameworks are fine ).
All game engines are allowed including Asset Store packages and open source frameworks. Web based games are also welcome. You can make a game for whatever platform you like, but know that if your game is Linux / Mac / or VR exclusive you're going to have a limited audience for ranking. But you do you.
Thanks for the question!