I think it is fair to say it is "most similar" to d20 Modern.
A bare-bones Grim Tales would run a "heroic normals" game. Then you just start adding back the components you need based on the genre you want to run-- magic, fantasy feats, guns, vehicles, etc.
How easy would it be to bring the following into Grim Tales?
- Blood and Fists
- Elements of Magic: Mythic Earth
- Psychic's Handbook using Wild Talent and the Psychic Advanced Class
- Hot Pursuit