A wiki a page can links to others one in the same wiki, this is known as a 'wikilink'.

The wikilink sintax in GXwiki is:

 [[  Page name { | description } ]]

If 'description' is present, it will appear in the link instead of the Page name. For example both links below go to the same page, but the second one have description:

without description:  Sandbox ,  with description: use play page to experiment without breaking anything

There are two ways to define a wikilink, directly typing it or using the 'wikilink' button in the toolbar. In this case a form will appear to help you choose the link from the existing pages.

If the wikilink points to a non existing page, a question mark will appear so you can create it clicking it.

See Wikilink in Wikipedia.