1.
Naquadah. Relatively free power. Oil is gone. Big oil is bankrupt in a couple months, and the global economy nose dives with it. Although they can't possibly have enough
Naquadah to keep 5 billion people clement all year around if they had enough problems trying to keep their first warship's lights on. Goul'd worlds are small. Thousands of people, not billions.
2. Rings. Free/cheap global instantaneous transit. More bankruptcy in the freight and transportation industries. Watch the world go down the toilet.
3. Colonization. 3 billion people leave earth in the first ten years. The worlds systems are in place to support 5 billion, from the helpfulness of 5 billion. The world falls apart because the people who know how to do things have gone, and the people who make babies are gone too.
4. Stargate connecting to Garbage planets. I assume the Stargate would be in space or in orbit. In orbit of a sun? I suppose the collapse of the landfill industry wouldn't be awfully cataclysmic, and it's somewhere to put Garbage Island, but the hippies are going to scream about being green intergalactically.
5. Religion. Jesus was a Goul'd. Cope with that. Oh shit. Has every Pope been the same Goul'd since the Roman Catholic church began?