Dormant Gomazoa's last ability triggers when you become the target of any spell, including one you control. It doesn't matter who else or what else the spell targets.

A spell can target you only if it specifically says the word "target." One that says "You draw two cards," for example, doesn't target you.

You become the target of a spell when a spell is cast targeting you, when a spell has one of its targets changed to you due to a spell or ability like Deflection, when a spell that's targeting you is copied and that target isn't changed, or when a spell is copied and one of its targets is changed to you.

Dormant Gomazoa's last ability won't trigger when you become the target of an ability.

If you become the target of a spell, Dormant Gomazoa's last ability triggers and is put on the stack on top of that spell. Dormant Gomazoa's last ability will resolve (allowing you to untap it) first.