vicious, cruel, severely, extreme
狠 centers on harsh, unyielding severity: viciousness, cruelty, or an extreme degree of something. The meanings all point to a lack of gentleness or moderation.
狠 combines 犭 (dog/beast) with 艮, which likely contributes a harsh or stubborn sound and meaning. The exact historical development is uncertain, but the character suggests a beast-like ferocity.
A wild beast 犭 beside a stubborn, glaring shape 艮: imagine a vicious dog snarling with extreme cruelty.
For ガン, picture a vicious gang of wild dogs: gang -> ガン, and their cruelty is extreme.