This sounds strange, it's ususally the other way around. If XP can read and right to the shares then it sounds like samba is setup properly and is working as it should.
It sounds like it maybe on the Gentoo side of it. Is there a firewall running on Gentoo, I don't use Gentoo but had a similar problem with SuSE when I was using it, the firewall didn't allow for samba ports It would allow me to see the shares but not allowing read/write access.
If the firewall is running, shut it down and see if that fixes it, if it does you need to add the samba ports to you firewall to allow access to the samba.