You can find a list of stock items in the gtk+ documentation[2].
With each item you will find the version of gtk+ in which it was
added. (Unless it has always been there since 2.0.)
In that list I don’t see `gtk-link` at all, even though it
includes the latest icons from 2.8. In fact, Google doesn’t to
have seen this term, ever. So I’d say the likelihood is high that
some other library or component on your system has added it. Or
maybe it’s part of your icon theme? Or something like that. In
any case, it has nothing to do with the canon.