Add the tag <%GT::Template::dump%> and you should be given a list of tags available on that page - they vary a lot by page.
In fact, the best way to do this is to create new user and then put
<%if Username eq 'NewUser'%><%GT::Template::dump%><%endif%>
in the footer template (replacing NewUser with the name of the user you've just created).
Then to see the full list of tags you can just log in as that user.
In fact, the best way to do this is to create new user and then put
<%if Username eq 'NewUser'%><%GT::Template::dump%><%endif%>
in the footer template (replacing NewUser with the name of the user you've just created).
Then to see the full list of tags you can just log in as that user.