As promised, I would write here whenever I succeeded to make encfs work together with GMailFS. Because Google is nice, but I don't trust them enough to have my files over there in readable format.
I tried encfs back then in October 2007, but that didn't work out. That was encfs 1.3.x. In January this year, version 1.4.0 was released and currently the latest version is 1.4.1.1. The ebuild is to be found in Gentoo Bugzilla.
I tried this version again on a pristine, mounted GMail filesystem and things got better now. A simple
would do. However, I got the following messages when actually trying to mount the encrypted filesystem after setting my password:
fuse: if you are sure this is safe, use the 'nonempty' mount option
fuse failed. Common problems:
- fuse kernel module not installed (modprobe fuse)
- invalid options -- see usage message
I don't know why it says that, because the filesystem is actually empty. So a modification on mounting did the trick:
(passing the nonempty mount option to fuse). No more Permission denied errors and the like, it seems to 'just work' 
Let's see if this works nice together with rdiff-backup, to have a backup of my /home at some external site.