I'm at the moment a Gentoo user. I was very curious of CRUX, so I tried it a few days and I decided to switch to Gentoo. I'm still discovering Gentoo, so I don't know every differences.

First by installing some software on CRUX I had the problem a more recent version of gcc was needed. Unfortunately gcc isn't updated so often in CRUX, so I had to create my own package. Like z3bra told it, it's very easy to write a package recipe, but I think it's much better to update the software. Keep in mind I come from Archlinux and I'm discovering source based distro. From my point of view, In Archlinux gcc is updated when a new version is released, so I think it can be done. I decided I want a full rolling release so CRUX wasn't a good choice for me.

Then when you want other compile flags, you have to write your own package recipe. In Gentoo a package is written and each user can configure the package such that it fits his needs. That's the idea of the USE flag. It's more complicated to write a package, you lose the simplicity of CRUX, but after that the users don't have to write a modified version of the package. For example you want a graphical version of emacs, you have only to activate the "X" flag. You don't want any image support in emacs ? You can disable the support of jpeg or png for example. So you choose what you want.

For me it was necessary to write some ebuilds for Gentoo (the package recipes), but you have much more packages than in CRUX. Depending on your needs, perhaps everything is already packaged for you.

Last thing I discovered is that you can easily change the compiler version. For example if you have some softwares which needs python2 and some python3, you can install both version of python. /usr/bin/python is a link pointing the the version you want. You can also easily switch between python2 and python3. It's the same for gcc for example.

You have now a point of view of a Gentoo user ;-)