geogebra is indeed not in the Entropy repositories. However there is an ebuild for it in Gentoo Bugzilla Bug Report No. 386239
so you could try installing it in a local overlay using the Portage package manager. An example of how to install a package in a local overlay to the main Portage tree is given in the post Re: Augmented reality
Alternatively, if you feel uncomfortable doing that, you could file a request for the package to be built and added to the Entropy repository -- see the post Requesting Packages for Entropy - New Policy
for the procedure. Don't forget to refer to the above-mentioned Gentoo Bugzilla Bug Report if you do.