Gaea's Revenge's last ability applies to all nongreen spells and abilities from nongreen sources, including ones you control. For example, you can't target it with the equip ability of an Equipment you control (unless the Equipment is somehow green).

Gaea's Revenge's first ability works only while it's a spell on the stack. Gaea's Revenge's last ability works only while it's on the battlefield.

A Gaea's Revenge spell can be targeted by spells and abilities that would counter it. The part of their effect that would counter Gaea's Revenge won't do anything, but any other effects those spells or abilities may have will still happen, if applicable.

If a spell is one or more colors, and one of those colors is green, that spell can target Gaea's Revenge.