Why can't we have our departmental pages on GCpedia that aren't accessible to other departments?

All pages on GCpedia can be seen by all users. GCpedia administrators can protect a page from editing if there is a strong justification for doing so (i.e. protecting a template that is used thousands of times on the wiki).

If you need your page protected to prevent other users from editing it, ask yourself whether it belongs on GCpedia in the first place. A simple alternative to protecting a page is to use the template {{protected}}, which asks users to refrain from editing the page.