There's a guy at the entrance that randomly appears and sells you GP, I think 100 gil for 1 GP. He appears about 10% of the time. If you have enough gil to buy a lifetime pass to the Gold Saucer, buy it and then just run in and out until the guy shows up.

The trick to earning BP is to choose bad slots that negatively affect you. Choosing poison when you have the ribbon equipped will make winning easier but won't get you a lot of BP. I recommend choosing the various materia breaking slots, or the HP or MP reducing slots.