One common theme among Dwarves, whatever the lore, is that they tend to come off as a bit rough around the edges to the other races. While watching the various Hobbit movies (and rereading the book, which I did before watching the first one), almost every time the dwarves first met another race I couldn't help but comment "-2 Charisma!" based on how they acted.
-2 Charisma is one of the dwarven racial templates in D&D and Pathfinder.
But there's a problem, GURPS doesn't have a "Low Charisma" disadvantage. You can be particularly charismatic, but there are no given rules for people who are naturally LESS charismatic than their IQ would indicate. Sure, there's shyness and low empathy, which are both similar in varying degrees, but they aren't QUITE the same.
But then, the penalty for dwarves might not be charisma (as it's defined in GURPS) based at all.
I'm thinking it might be better represented as an odious personal habit or a negative reputation. Dwarven Gruffness, either with the limiter that it only bugs non-dwarves or those unused to it, or treated as a reputation with anyone that isn't a dwarf or used to dwarves.
I have thoughts for posts but no motivation to write them.