To make thing clear about Yeti3D :
There's a GPL version (V1), that means that you can use the code, modify it, etc... provided that you release the modified code under GPL too. This version supports rendering of worlds made of variable-height walls, floors and ceilings that you can build from the bundled editor. There is no support for weapons, AI or whatever.
V2 version is more complete : enhanced editor, support of "sprites" (think about Doom's characters) that can be imported MD2 models. It should be released under GPL too someday.
You can use V1 without asking to the author. If you think you need V2, then you have to ask him for the moment.
You need to understand that unlike Doom/Quake/Half-Life engines, it is not a FPS engine where you can only add some material to make your own FPS. You need to have good coding skills, as you will _need_ to get your hands into coding. Understanding of the basics of a 3D engine is also required, in my opinion.