If Guardian Angel is attached to a non-unit Brutalizer (Brutalizer was a unit thanks to Orianna, Mechanical Marvel's ability but is now back to being just a gear) and Brutalizer gets killed by Disarming Rake, does Guardian Angel's effect apply and die instead of Brutalizer?

Yes — Guardian Angel's replacement effect still applies, and the Angel dies instead of Brutalizer, even though Brutalizer is no longer a unit.

Attachment doesn't care that the host stopped being a unit: Guardian Angel stays attached, its printed Equip line goes Inactive, but its Effect Text ("If I would die, kill Guardian Angel instead. Heal me, exhaust me, and recall me") stays active and is appended to the Top-Most card's rules text — Brutalizer here (Rules 718.3, 136.2.c). Disarming Rake's "you may kill a gear" kill instruction on Brutalizer is a qualifying "I would die" event, so the replacement intercedes: Guardian Angel is killed and goes to the trash, and Brutalizer survives.

The follow-up instructions just have little to bite on — Brutalizer is a non-unit gear, so "heal me" does nothing meaningful and "exhaust me / recall me" only matter if it happened to be exhausted or sitting at a battlefield (unattached non-unit gear there would be recalled in the next cleanup anyway, Rule 459).

Note the flip side: if they point Disarming Rake at Guardian Angel itself, the Angel just dies and Brutalizer is unaffected.