The installation of the Cubefree package follows standard GAP rules, see
also
Chapter Installing a GAP Package in the GAP reference manual.
So the standard method is to unpack the package into the pkg
directory of your GAP distribution. This will create an cubefree
subdirectory.
To use the Cubefree Package you have to request it explicitly. This is
done by calling LoadPackage
like this:
gap> LoadPackage("cubefree"); true gap>
Cubefree manual