[erelde]

Aka more simply as H2G2, "the" and "to" aren't required in an acronym.

[doc_gunthrop]

Except nobody ever refers to it as "H2G2".

Using acronyms already adds some degree of ambiguity; shortening a commonly used acronym makes it basically unrecognizable, even when used within the proper context.

[asavadatti]

H2G2 is a pretty common acronym. In fact I've only heard it referred to as H2G2 or Hitchhiker's Guide.

[erelde]

On its wikipedia page its referred throughout as H2G2.

H2G2 9 times

HG2G 2 times

HHGTTG 2 times