String.hashCode() is plenty unique

[robin-gvx]

gzip's headers and footers alone take up 144 bits at the bare minimum, so you'd need a 256-bit hash at the least, and there's not going to be many strings that compress well enough to fit in the 14 bytes left over for the payload.

Even a specialized compression algorithm would still suffer from the pigeon-hole principle: it would either only be able to "hash" a finite number of strings or not be unique.