mrifk is a Glulx → I6 decompiler, but mrifk is somewhat out of date. Even if it were up to date, it would produce ugly I6 with meaningless variable/function names (this is inevitable… the original names are no longer available).
There are still isolated bits that would be pretty easy of change, like switching some given bit of literal text with something else of the same length. Most things would be much harder.