Gus, although not one of my favorites from Old Possum's Book of Practical Cats, is an interesting character nonetheless. After reading this summary, I would suggest watching the excerpt from the movie adaptation of Cats, or at least listening to the musical version of the song. It's worth it.
Gus could always be found at the theatre door. His name is really "Asparagus," but too many people couldn't pronounce that, so his nickname became "Gus." Gus is an old cat: shabby, thin, and shaky. He was supposedly quite a cat in his youth, though. The exploits of his old days are what Gus likes to talk about with his friends, especially his adaptation of "Firefrorefiddle, the Fiend of the Fell." He boasts of the roles he played in his prime. And if someone gives him a bit of gin, he'll go on and on about roles he doesn't usually speak of. He is "old school" theatre (as it were), and he doesn't trust these young lads that "do not get trained." He thinks that naught can compare to his greatest role (which he repeats many times) and admits that "Theatre's certainly not what it was."