It appears that whether the show will continue or not has yet to be determined. Not much to say about new episodes after Rodent Rebellion, but no official word about it being the series finale either.
Therefore, as long as we don't have an official word to say otherwise, it would probably be best to consider the show to be on hiatus, and not prematurely write it off as cancelled. Will there be more episodes of TGS? Will Garfield go on to a new show of some sort? Let's just wait and see.
For at least some ideas about the show from someone who actually works on it:
Vital Orange Cat News - Indicates, but does not prove, that the show is cancelled.
Mushroom Soup Sunday - Notes that "a script about a lazy, lasagna-eating cat" is being worked on; no other details about it are given.