If you're running KDE, then you can use kxconfig to change the settings. Otherwise, grep mouse /var/log/XFree86.0.log (or similar) to see what happened when you started X, then edit the mouse input sections of /etc/X11/XF86Config-4. Gentoo may have another way of doing this however...
Also, do a search of this site - there are many, many threads dealing with exactly the same problem