Doom: The Golden Souls 2 is another interesting free GZDoom game, taking some inspirations from Mario. Yes—really. It's the same developer behind the Castlevania: Simon's Destiny fan game.
Here's what the they said about it:
Doom : The Golden Souls 2 is the sequel of the groundbreaking mario styled hub-based mod, this time you'll travel through levels with a world map, unlocking new paths by reaching secret exits or by collecting big coins. You must retrieve 7 Golden Souls stolen by the Cyberdemon's minions in order to save the world... once again!
To run it, you need GZDoom v3.4.1 at a minimum according to what the developer noted in the download.
Testing it out myself, it's actually not bad, somewhat amusing in fact. A little weird though, since you're fighting demons in a rather colourful setting with cheery music.
Hat tip to nate.