I purchased a gold pass using the easy pay option. There is only one payment left. Could I use it to enter the park or would I have to wait until it's completely paid off?

The pass can be used, but needs to be processed (unless it was a renewal).

Note: You cannot pay the "easy pay" at the front gate, they just will not allow it.

You can use your pass while your making payments,thats the reason they have this option.Like making car payments,you still get to ride it! You can also get in once without processing if the line outside is to long,do it later in the day when its shorter,worked for us.