welcome in the wonderful world of sabayon/gentoo, because here is no apt/dpkg mechanism and you using mint for a longer time, it is better to install a clean new sabayon iso, this is using USE-flags for stuff, eg. global desktop etc.
best way i think is to look which stuff is needed/missed by you and have a look at packages.sabayon.org.
if there are questions, go to irc to become help
cu, hf stephan