My 200 GByte SSD on my Gentoo system is 99.5% full (sometimes hits 100%,
but I've been erasing as much as I can).
Whoa, that's a ton of space being used. The Gentoo installation on my workstation takes only 20G, but that's with /home on a 1TB HDD array. So a >200G Gentoo installation seems, well, oddly bloated.
I thought I could move the /var from the
SSD to HDD and open some space, but I'm having some trouble.
I'm curious why you chose /var as the directory to move. It doesn't seem like a good idea to me because there are programs that may run early in the boot sequence that try writing to /var/run or /var/lock, or reading from their "home" directories in /var/lib, before /var itself is mounted. The logger itself may also try to write to /var/log before /var is mounted, either screwing up the mount, wiping out the early boot log, and/or leaking hard drive space on the root that gets hidden when it's overmounted. Finally, /var typically doesn't take up very much space, so the benefit of moving it doesn't seem that great.
Is your /var directory taking up >1G? What subdir(s) of /var are huge? If /var/log, then there's a really good first place to start recovering space. If /var/lib, track down the offending daemon; mysql is one of the the usual suspects. If /var/tmp/portage, then just wipe that thing out, and if you have the RAM, replace the /var/tmp/portage with a tmpfs. You never need to keep it between boots.
Are there other directories under / that are taking up more space than /var, and might benefit more from moving them off the SDD? /opt tends to be a prime offender if you have several binary-only packages installed on your system. If you don't clean /usr/portage/distfiles regularly, then it's probably eating up a ton of space, and I would think you'd recover much more space, much more safely, by cleaning up and moving /usr/portage/distfiles to HDD than you would by trying to move /var.
tl;dr: figuring out why /var is so big, or moving something besides /var, is probably a safer and more maintainable solution than trying to move /var off the root filesystem.