I'd generally handle it (whatever game or sport it is) with an opposed gambling roll. However, that really isn't that exciting. Especially, when you consider several things involved with gaming, betting, gambling and determining the odds. This introduces possible options that can be included that don't just hand a player with a good gambling skill a big pot of free Gold.
I'm relatively familiar with RMSS, and that has a huge wealth of options you can introduce to make the gambling skill more interesting.
Quite often making a gambling roll might just be used to determine whether a character it's a good idea to play or not, and also determines their ability to restrict their own addictions!
Let's say it's *just* a game of cards. One vs One over a table with an unmarked set of cards.
First complication that could arise. Are both players equally familiar/experienced with that specific game?
In theory, you *could* make the gambling skill like a weapon skill and split it into several categories each relevant to a different type of card game ... however, that's a little too involved, even for Rolemaster...
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However, it's often simpler to use another skill to determine this. Say, like Culture Lore. If the player is from a different culture than the game then if they fail the culture lore roll they get the appropriate penalty to the opposed gambling roll.
It can be made into a far more complex answer, but the quick answer is that when gambling a player shouldn't *just* rely on the gambling skill alone there are other factors which allow for differentiation between the types.