Hexes are a standard measurement for combat in GURPS. "One hex on the combat map represents one yard of distance. It is also the basic unit of movement: each hex a fighter moves represents one yard of movement. The number of hexes you can move on your turn depends on your Move score and your maneuver."
Things that effect an area (explosions or area spells) work as following.
An area of effect of 1 covers only a single hex while an area of effect of 2 covers one hex and all adjacent hexes. Each increase in the area of effect adds even more adjacent hexes. So AoF 2 covers 7 hexes (center hex + 6 surrounding ones, AoF 3 covers 19 hexes (7 hexes+ 12 surrounding ones), AoF covers 37 hexes (19 hexes + 18 surrounding ones) and so on.
The formula works out to be 3r^2 - 3r + 1
Something that explodes does full damage only to the center hex. For everything else in the blast radius roll this damage but divide it by (3 x distance in yards from the center of the blast), rounding down.
- Basic Set 384