So far as installing Gentoo to begin with, it comes with the base files all on the CD. You sync with the remote server / download more files toward the end of the install, that's all. If you have the files you need downloaded somewhere already, you can use them.

When you install pacakges, portage doesn't have to fetch the files itself itself at the time you build the package, no. So long as the files are in the right place, portage will find them and use them. If the files aren't there, then portage auto-fetches them, either from a Gentoo mirror or, if it can't find the file on a mirror, directly from the developer's site.

<b>emerge mozilla-firefox</b>, for example, is just shorthand for <b>ebuild fetch, ebuild unpack, ebuild compile, ebuild install</b> and <b>ebuild qmerge</b>. You can manually do the steps yourself. If you just want to fetch, you can do

<b>ebuild /usr/portage/net-www/mozilla-firefox/mozilla-firefox-0.8.ebuild fetch</b>

And it'll fetch the right file(s) (using wget I believe) and stick the file(s) in the right directory, and then later you can do the whole emerge. Or you can manually download the file yourself and stick it in the right folder, which would be slower and more hassle-ish. Doing the steps manually is also helpful if you want to modify the source yourself before portage compiles and installs it. I sometimes need to do a manual fetch and unpack, fix some code, then do the rest. Portage is flexible.