According to this
website your printer should be supported from hplip version 1.74.
There are two ways to solve this. Either to install hplip-1.7.4a-r1 or hplip-2.7.7.
1) Install hplip 1.7.4a-r1 by opening up a konsole and type "su" (enter root password).
Then type "emerge =hplip-1.7.4a-r1"
2) hplip-2.7.7 is no longer in portage and anything above version 2.7.7 needs to rebuild udev-1.1.4 which is not desirable. So go to this
website and download hplip-2.7.7.ebuild (by right-clicking and "save as") to the desktop. Now open up a konsole and type "su" (entering root password).
Copy hplip-2.7.7.ebuild to /usr/portage/net-print/hplip (E.g. cp ~/Desktop/hplip-2.7.7.ebuild /usr/portage/net-print/hplip/hplip-2.7.7.ebuild)
Next type "ebuild /usr/portage/net-print/hplip/hplip-2.7.7.ebuild digest"
Next type "emerge =hplip-2.7.7"
After is has finished installing you will probably have to type "etc-update" if it tells you to.