Meta ebuild policy
The GNOME project maintains a meta ebuild named gnome-base/gnome. This ebuild is intended to install all of the packages considered by the upstream GNOME project as being part of the GNOME Desktop. It is not intended to be customizable, except for a few exceptions: hardware customization and accessibility. This is the recommended method of installing the GNOME desktop on a Gentoo system.
The primary alternative, if you require specific GNOME programs, is to emerge them individually. If, for example, you want to use Evolution for email, you should emerge evolution. This should pull in any necessary dependencies to run evolution.
There are many subsets of GNOME that can be installed and usable. There is no way that the GNOME project can support them all, or any significant fraction of them. We do make one such subset available: gnome-light. This subset is intended to include the programs necessary for basic desktop functionality without any additional applications. It basically includes gnome-session, gnome-shell, gnome-terminal, nautilus, and yelp. Gnome-light can be used as a jumping-off point for installing intermediate subsets of GNOME between it and the full gnome meta ebuild, by emerging extra applications as necessary. No other subsets, including making the GNOME ebuild more customizable with USE flags, will be supported.
Upstream GNOME does not support any advanced CFLAGS beyond
-O2, and neither does the Gnome Herd. Unreproducable bugs with any CFLAGS beyond
-O2 and appropriate
-march are liable to be closed as INVALID unless they can be reproduced with valid CFLAGS.