As you know, the wiki gardener is a highly beneficial role to the efficiency and effectiveness of your intranet.
You asked what is this person's (or group's) actual tasks?
To answer, I found this great blog post on the subject called: The Wiki Gardener: Tasks and Requirements
It lists tasks as follows
- provide requirements for a wiki adoption and create a useful application
- practice systematical monitoring of the new and varied content
- if applicable, and according to internal wiki policy: integrate contents out of the comments on relevant pages, as long as the content know-how is available
- delete comments, when the elaborate solution has been taken over in the page content
- undertake cosmetical tasks, so that the wiki does not grow exuberantly with weeds (typing errors, grammar and spelling, incomprehensible verbalization etc.)
- repair and insert links in wiki pages
- catchphrase, tagging and hierarchical integration of pages
- improve clarity and legibility of pages
- possibly secure consistant editing of pages
- create examples for other users, how the wiki can be used