I'm phone posting and its a bit shippy but here it goes.
It's an AU where Zim gets exiled from Irk like the start of the show and winds up finding a prehistoric version of Earth to invade. Zim's Voot Cruiser suddenly malfunctions and ends up crashing landing there. Meanwhile Gaz, the most skilled amazon hunter in her tribe witnesses Zim's ship crashing down while on a night hunt, she investigates thinking it's a shooting star that fell from the night sky.
When Zim and Gaz encounter each other they have a quick fight only for Gaz to beat and capture Zim with her superior hunter skills. Gaz is actually interested in Zim's weapon, gadgets and lasers he used to battle her, thinking it's some sort of magic and mistaking Zim as an almighty Star God who fell from heaven. Gaz then decides to take Zim as her "mate for life", thinking he'll have her give birth to powerful demigod children. Lots of snu snu ensues.
There are ideas for lots of crossovers from other shows in this AU such as Mandy from Grim Adventures as an amazon from a rival tribe who Gaz competes with and Penelope Spectra from Danny Phantom as an exiled witch who sacrifices humans to her dark god.