The use of the const keyword in C++ is not always clear. You might not realise that const Something* declares a pointer to a const Something. The pointer can be changed, but not the Something that it points to.
Therefore, the RefPtr equivalent of Something* for a method parameter is const Glib::RefPtr<Something>&, and the equivalent of const Something* is const Glib::RefPtr<const Something>&.
The const ... & around both is just for efficiency, like using const std::string& instead of std::string for a method parameter to avoid unnecessary copying.