Actually, in Gentoo, you can get a kernel.org kernel without any patches.
There are a variety of kernel packages available. Here are 3:
To use the kernel patched by Gentoo:
To use the latest released kernel from kernel.org
Or, to use the daily git snapshot from git.kernel.org
Or, as with any distro, you can just download the tarball from kernel.org and have fun. Gentoo (potentially) makes it easier because it is required to compile the kernel yourself anyway.
In addition, make sure that you know what areas of the kernel you want to hack at. Honing your focus will help a ton. The kernel is a big piece of code!