Luck in general takes away skill in games, thats why games like snakes and ladders are boring for adults(atleast without stakes of some sort), and games like risk are more popular as you grow up and start understanding strategy. I think luck in general is bad for competitive games, for instance look at SSBBrawl, they added tripping specifically to try and make the game less competitive. Why do you think there are no damage ranges/crit chances in SC2, because that adds randomness and takes away from skill.

Now randomness can still add facets of skill, such as randomed positions, it gives mafia people to pretend to be, and towns folk ways to rule out peoples stories, that is a good type of randomness as well as adds replayability to the games(such as random maps and starting positions in SC2). Now if you want a gambler to add this kind of stuff to a game, you have a very hard task ahead of you. Even after you design a character you have some more problems:

First you have to ask is it fun to play, gambling often is fun and is rewarding when you win and tends to be fun with proper risk vs reward.

Then you have to make sure it isn't more unfun to play against then it gives fun to the player, you don't want somone that just ruins the game because he is unbalanced or ruins someone elses role.

Those are two very important things to think about if you want to add someone into the game after you cover the whole is it skillful and does he add anything to the game portions.

My personal suggestion would be to have the gambler deal with points as suggested but have him be a neutral, who gets points for killing/harming one side but then winning with the other side, so if he kills 4 towns but he still wins with town he gets more points then if he just wins with town. Just a basic idea i came up with on the spot, but i think it fits the role of a gambler(go as far as you can and still pull off winning against the odds)