I play Acceleration Gate on my turn. My opponent is in combat with my Bilgewater Tracker. If my opponent uses Decree of Focus, does my opponent get +4 might for the Gold unit, or does it get +1 might only?

No — nothing gets added here, because your opponent can't legally play Decree of Focus in that combat at all. Decree of Focus needs its chosen friendly unit to be in combat with an enemy Fury unit (or chosen by an enemy Fury spell). Bilgewater Tracker is a Body unit, so it doesn't qualify as the Fury unit, and a Gold token is a Colorless gear, not a unit of any domain (Rule 148 — gear and units are separate card types). Neither piece of that combat makes the spell legal.

Acceleration Gate is a red herring: readying up to 4 units, gear and/or runes doesn't change a card's domain or card type, so it can't turn the Tracker or a Gold token into a Fury unit.

Two clarifications on the numbers: Decree of Focus is a flat +4 Might when it does legally resolve — never +1, it doesn't scale by domain or anything else. And if there's a real Fury unit at that battlefield that I'm not being told about, the +4 goes to your opponent's chosen unit, not to your Tracker.