I did SEA-SFO-HNL-GUM-MNL as an MR last year. IIRC, you only need to go through US customs & immigration on the way back when you connect in GUM. There's no immigration in HNL in either direction as you are coming from the mainland on the outbound and from a US territory on the inbound and already cleared immigration when you entered GUM.

Of course, you have to get the stamps in MNL on entry and exit.

Oh, and don't fret about missing your GUM connection if your flight is late. Our flight was 45 mins late and UA held the other flight so connecting passengers can make it.