MATE

MATE is a fork of GNOME 2 by The MATE team, which is available in the Portage tree; according to their manifesto they aim to keep a traditional desktop, maintain an open development model, have an open relationship with GNU/Linux distributions and also serve as a good alternative for lower end hardware.

USE flags
First enable and/or disable the USE flags you want for mate-base/mate.

Emerge MATE
Then, to install the MATE desktop environment, you can run the following command:

Usage
You can either use a login manager (slim, gdm, ...) or start it manually using startx.

To make the login manager work, you'll need to specify in the configuration of the login manager that you want to start the MATE session (mate-session); some allow you to do this interactively, for others you will need to do this in the configuration of the login manager. The default session can also often be changed by setting XSESSION="Mate" in.

To start it manually, you need to create ~/.xinitrc and make it contents as follows:

Does MATE rely on a specific service manager or init system?
No, MATE has been tested to work with both OpenRC and systemd and might work on other service managers and init systems too (untested, but no known reason for it to break); systemd support was added in release 1.6.

Can MATE be installed side-by-side GNOME packages or do they block?
As the MATE packages use their own categories, it is possible to have MATE and GNOME 3 installed at the same time which allows you to test either; taking it even a step further, if you change MATE to not have a top panel (as it gets hidden under the GNOME 3 shell) you can even start mate-session within GNOME 3 and run MATE and GNOME 3 at the same time.

GLib-GObject-ERROR: object GsmAutostartApp 0x73ca40 finalized while still in-construction
When you get this error (see ~/.materc-errors), it is usually preceded by a warning, fixing the warning could fix the problem; for example, when I get to see:

mate-session[881]: WARNING: Could not parse desktop file /home/username/.config/autostart/some-naughty-broken-program.desktop: Key file does not have key 'Name' mate-session[881]: GLib-GObject-ERROR: object GsmAutostartApp 0x73ca40 finalized while still in-construction

In this case, you can resolve this by moving away the desktop file or fixing it up by adding the Name key. If you want a clean start, you can move those files out of the way by backing them up: