[Yip1982]

What does Gus mean when he says "I knew how to let the cat out of the bag?" Does this mean he's good at revealing secrets or good at literally doing this?

[Alonza0]

Revealing secrets. It can be assumed that at one point he had to act out revealing a secret (perhaps accidently). The lyric is intended to be slightly humorous, as the saying refers to a cat. It's obvious that some of the kittens aren't familiar with the saying, as, when Gus sings this line, some of them can be seen looking confused or tugging at an elder cat for clarification.

[Drumdraper]

I think that T.S. Eliot added that line into the poem for humor. It's a common phrase about revealing secrets or information and since it pertained to a cat,fit into his rhyming pattern, and was funy, he wrote it in. If you see a show live this is usually the first part of Gus's song that the audience laughs at.