If Portage says the package is masked or unavailable:

emerge --sync

Then retry:

emerge -av dev-util/codeblocks

emerge -av sys-devel/gcc sys-devel/gdb

emerge -av x11-libs/wxGTK

codeblocks

codeblocks --version

echo "dev-util/codeblocks ~amd64" >> /etc/portage/package.accept_keywords/codeblocks

emerge -av dev-util/codeblocks

dispatch-conf

or

etc-update

Run after autounmask changes.