The word "nonsense" suggests that Gregor's situation is rubbish.
Samsa's recent transformation into a vernim, along with the appreciation of the unchanging room he had in his human form and the dull weather, made him get to a point where he determined it was all nonsense, it was worthless and nonsensical, that is to say, rubbish. He wasn't enjoying it that much, he wanted to sleep a little longer to forget all the nonsense.