Quote Originally Posted by Wulf202 View Post
Connect red to black and put the white in with the other whites. Youll have the spare yellow wire nut the pool guy put in. Fire it up just long enough to check the light. If the pool light is out of the water it will cook the gasket

The gfci will trip the instant the power is on and water hits hot wires. That's why its there. It might tingle for a half second at worst. Not much of a safety concern.
This. Picture sums it up perfectly. Red is your switch leg coming from the switch box. Was capped to prevent shorting. The center conduit, I assume, is going to the light. Those two wires should have been capped individually.

Before you do anything, you could verify this all, by hooking up the switch. In the on position, you should be able to read 120V on the red wire in this new jbox. Once you are sure that power is reaching this part, then you could proceed to put everything together. Hooking the red to the black on the center conduit and tying the common in with the other commons should get power to the light.