'root' access means you are the user with uid=0 - you have absolute control over the virtual machine with the exceptions that:
*you have to use one of the Linode kernels
*you cannot load kernel modules (but Linode will respond favourably to reasonable requests to build features in, subject to stability and security conditions being satisfied)
You can either update the supplied Gentoo distribution or build your own from the tarball of your choice.
Compiling things like glibc puts a big strain on a small Linode - most people either use distcc or build their systems on home machines.