Sure, all swap is is another partition basically solely used for swap.
You need to find out what /dev/hdX it is, and then supply that to Gentoo as well.
If you are real gung ho, I read an article once (that would take a little searching on www.google.com/linux
for) about using the same swap for all installed LNX distros, and for winbloze as well. Probably not worth effort, but still apparently not too hard (probably by specifying in winbloze to use the same /dev as you did in LNX, that would make sense)