Gambling:
I like the DC method, and from this point forward I will work it as such.
The DC will be set at the onset, to represent both the stakes and the skill of your toughest opponent.
Using Profession (Gambler) for your roll drops the DC by 5
Having 1 rank in Bluff, Sense Motive, Perception, or Sleight of Hand grants you a blanket +2 for each one. At 10 ranks they become +4
If you don't have Profession (Gambler), you can use any one of the synergy skills instead, but don't get to use the synergy for having that skill (i.e. you can't use Bluff, and get a +2 for having a rank in it).
The amount you win will follow the method before of [your check result divided by 5, and then multiplied by the amount of your bet]. That's how I see it, if you want to add anything, just let me know and I'll see how I would want to work it in.
To bet on events you're not participating in, it is just either a simple Int, Sense Motive, or Perception check. Synergies for the skills apply. This way, with less benefits tied to the betting on, you have a chance of winning more by participating.
If you fail the DC, you lose your bet, if you fail by 5 or more, you lose the total of your bet extra. I'm not putting a specific 1 or 20 rule, since 1 is minus 10, and 20 is plus 10, you already win or lose more by getting the penalty/benefit.