I'm new to GOTM and I keep hearing about different classes
open, conquest and predator. what's the difference between them

Can I direct you to gotm.civfanatics.net

Have a look around that website, and if you still have questions about the GOTM process, come back here. :)

In answer to that specific question: Open class is a 'standard' difficulty level, using (basically) the default rules. In 'Conquest' class, the player is usually given a few benefits, such as extra gold, extra units etc. In predator, the AI is given extra benefits, such as lower unit support costs.

Overall, its a handicapping system to allow people of varying skills to compete in the same competition. :)