Guide Number is about the Inverse Square Law, which describes the way the direct flash intensity falls off with distance. At 2x distance, the flash is reduced to 1/4 intensity. At 3x distance, reduced to 1/9 intensity. At 4x distance, to 1/16 intensity. Like your flashlight or car headlights also fall off with distance. So distance does overwhelmingly affect direct flash.
The Guide Number is a simple way to describe this easily, uncomplicated.
Supposing if the flash exposure is considered "correct" exposure at say 10 feet and f/8, then that defines this case of Guide Number as 10x8 = GN 80 (for whatever ISO being considered). So then we know 20 feet is properly exposed (assuming direct flash) at 80/20 = f/4, and 5 feet is exposed at 80/5 = f/16. Makes it easier than computing squares. Can use meters instead of feet. GN in feet is 3.28x GN in meters, simply because there are 3.28 feet in one meter.
The flash zoom from 105 mm to 200mm concentrates the wider 105 mm beam into a smaller 200mm beam. Concentrated into smaller area becomes a bit brighter, so it has a bit higher guide number (but the area coverage is smaller). This is a variable for the specific reflector design.