1. check your /etc/make.conf for something like:
	PORTDIR_OVERLAY="/root/portage"
if its not there, create this line. It tells portage to search in this portage tree, too.

2. mkdir -p /root/portage/app-misc/gregexp

3. mv gregexp-0.2.1.ebuild /root/portage/app-misc/gregexp

4. ebuild /root/portage/app-misc/gregexp/gregexp-0.2.1.ebuild digest
This creates the md5sum of the package and does some checkings for syntax errors and the like in the ebuild.

Now, you can build and install it with "emerge gregexp", please note that you need ACCEPT_KEYWORDS="~x86"

Note: This document refers to version 0.2.1 of gregexp. Please replace this version to the one you would like to use!