Instead of resizing swap partition you can create a swap file. Example:
(Create (almost) 1GB file filled with zeros)
dd if=/dev/zero of=/tmp/swap_file bs=1000 count=1000000
("Format" file as swap)
Now you can use free or top to verify that available swap space really increased. Of course the trick above works only until reboot - if you want it to persist, just add (on your Gentoo) server swapon /tmp/swap_file to /etc/conf.d/local.start.