Self leveling concrete IS used to level out areas as you suggest, but to do what you are thinking would be very expensive with that product.
A typical garage will have about 3" of slope minimum, usually more. Your options are:
Pour new concrete slab over existing with a minimum thickness of 2".
Wood framing, rip down 2X6 to match the slope and cover with plywood, this is known as sleepers. This method will allow you to insulate below the floor and have room for electrical, plumbing, or HVAC systems to be placed below the floor.
I know of no other options.