You have to pay attention to spaces

Select[NetExtract[lm, {"Output", "Labels"}],  StringContainsQ["hitman", IgnoreCase -> True]]

(* {" Whitman", " Hitman"} *)

The vocabulary token is " Hitman" with capital H and a space at the beginning. In the GPT tokenization the space is important as it allows to distinguish between at token at the beginning of a word from a token in the middle or at the end. You can see that is string correspond to a single token:

NetExtract[lm, "Input"][" Hitman"]

(* {49990} *)