I've been very happy with my switch to Gentoo. One of the cool things is that I can use the package manager for most development tools as Portage contains up-to-date versions of most of them. (Case in point, I just updated Node.js to a version that was released just four days ago.) All the more shocked was I when I found out that Atom isn't one of them and that information on how to install Atom on Gentoo is elusive.
Maybe because in the end the process isn't that complicated. Basically I just had to build from source, mostly following the documentation on how to do that. I'll still repeat it here to have it readily available when I have to update.
These steps worked for Atom 1.5.3.
The first three requirements listed in the documentation are a 32/64 bit OS, the C++ tool chain and Git. Every Gentoo install covers that.
Then come Node.js and its package manager npm, which comes bundled with it.
I unmasked all 5.x releases of Node (with
=net-libs/nodejs-5*) but you should get a sufficiently new version even if you don't do that - make sure to check, though.
Next on the list are the development headers for the GNOME keyring, which is the only non-obvious (and generally somewhat peculiar) step.
gnome-base/libgnome-keyring-3.12.0 fixed this for me.
▚Build & Install
These are just the install instructions:
git clone https://github.com/atom/atom cd atom git checkout v1.5.3 script/build sudo script/grunt install
For an install that is better integrated with the OS you might want to use a real ebuild.
There are a couple of Portage overlays containing Atom, most noteworthy the ones belonging to the Gentoo-based distributions Sabayon and Funtoo but also others. I opted against this solution because using the overlay of another distribution looks like a troublesome move to a novice and using "some guy's overlay" doesn't sit well with my general paranoia.
But you can also do it yourself - or so Till claims in the comments. If I find some time I'll try it and update this post.