How do I find my cars in GTA Online after I accidentally downgraded to a smaller garage?
If you had cars in a garage, and changed to a garage with a smaller number of spaces than cars you already own, then those cars that wouldn't fit in your new garage are gone. Completely deleted.
So if you had 10 cars in the 10 car garage, and you switched to a 6 car garage, 4 of your cars will be deleted. As far as I know, you can't get these cars back by buying the bigger garage again.
I'm not sure how the system determines which cars are deleted though. You can reorder cars in your garage so this may have some effect on which ones are deleted.