Pretend for a moment that you don't have Global Entry.

You clear immigration in GUM, and customs in HNL.

So on arrival in GUM, your passport is checked.

On arrival in HNL, you turn in the customs form. This is where you provide CPB an arriving flight number, and that flight number would be your flight arriving into HNL.

So, when using Global Entry in GUM, you're doing two steps - automatically verifying your passport for immigration purposes, and electronically filling out your customs form for customs purposes.

As you wouldn't give CPB your ICN-GUM flight number were you not GE, providing your GUM-HNL flight number, as the GE station defaulted to, is almost certainly the right answer.