Originally Posted by maxdustington
Did you test the old GFCI? If you still have it you should test and reinstall if it is still good. The new one could be defective, or be the wrong amperage. I suppose you could always borrow one from your washroom or kitchen and try it on your pool light circuit if they are the same amperage.
You could also check the resistance of the pool lights to see if one of them are damaged. It's not a conclusive test but if they are not all around value then you know something is wrong.
If you change multiple parts and then have issues, I always try to return it to original condition if possible and start to isolate components.
It could also be a loose connection somewhere. Maybe a connector on the lights got damaged from the water or disassembly/reassembly.
Old GFCI thrown out. Amperage should not be an issue, two 300 watt lights on their own 15 amp circuit, the outlets on the GFCI itself are rarely used and nothing is plugged into them, and the outlet works fine with other electrical devices.