I ordered a gasket when I replaced the incandescent bulb with an LED for my old Amerlite this past summer. I discovered that the new gasket, being sold now, is thicker than the old one. I could not locate one like the old one after going to several stores. The solution to using the new one is longer stainless steel screws. I was in a hurry to get the new color changing light in so I conditioned the old gasket with non silicone 303 Aerospace Protectant and cleaned up and smoothed the old face plate very well. Then I lubbed it very well. I would not recommend doing it this way.
Mine has not leaked but I think I'm very lucky. Until reading your post I had forgotten that I needed to take light bucket out, disassemble, and measure old screws to order new longer ones. I'm going to order a new faceplate too as the rim, where gasket fits is not in the best shape.
If I were doing it again I would certainly be more patient and use the instructions for proper replacement as the above posts. The color changing LED cost over $100 but I was willing to take the chance. There really isn't a good, valid reason to take that chance. My reason was not good and was not "valid". It was reckless and, as I've said, I've been lucky.
If you have an old Amerlite, or similar, bucket, that is old enough, be aware that you will (or may) need longer screws to accommodate the new gaskets being sold for that type of light. Purchase them in advance.
BTW... Over the years I've had the incandescent bulbs changed a few times by pool guys. We used to keep our pool light on all night. I never saw the inside of bucket till past summer. The plastic socket for bulb was a bit affected by heat. Also the old gasket had deposited a bunch of old rubber on the bucket and cover ring. It took loads of elbow grease and several different types of cleaners and scrubbers to get the old stuff off. That right there points to why you replace the gasket.
gg=alice