The OEM gasket is stuck to the OEM light. But I used the one that came with the replacement. I snugged up the bolts then dragged over the garden hose to test for leaks. Since I didn't have any leaks, I left it alone and it has been a non-issue all winter through tons of snow and a bit of rain (and the truck has never seen the inside of a garage).
I installed the replacement light this morning. I was concerned that the gasket that comes with the replacement was about 1/2 the thickness of the gasket on the OEM light. But once installed I wet down the light well and let it sit for a while. Then removed the light to check for any water penetration as you can not tell anything without removing it. It was dry inside so I guess the thinner gasket works fine. I have seen in some thread somewhere that when people have had water stains on their headliner and found the OEM light leaking, they used some gasket sealant material from Home Depot to fix it. But for now at least all seems good. Hopefully it will remain that way.
The replacement light does solve the issue with clearance on the OEM light with cameras for shells that do not have a cutout specifically for the light. The replacement protrudes less than 1/2 the distance from the cab that the OEM light did so the shell can be installed where it should be on the bed rails, lessening the chance of water leaking into the interior of the bed.
I did not yet do as you did and dismantle the OEM light to remove the two cameras and keep them attached to the connectors. I just taped up the connectors and stuffed them above the headliner for now. So far there are no error messages being given about the cameras being disconnected. The cargo bed view camera and the rear view mirror digital mode just display as black. If any errors to eventually show up, then I guess I can remove the cameras from the OEM light and reconnect them before stuffing it all back into the head liner space.
Edit: Ha! A few minutes after typing this, I got nervous about the truck eventually throwing an error with the cameras disconnected. I looked closely at the OEM light and saw I could easily remove both cameras. So I did just that and re-connected them to the wires and then put it all back into the headliner. So now the cameras are connected and hopefully the truck's brains will stay happy.
In another thread, Jimmy indicated how we could extend these cameras to the back of the shell, or even to the back of a trailer and still use them. It looks fairly straight forward to do with the parts he referenced. The only part that would be tough would be getting the wires out of the headliner in a way that does not allow water penetration.
New light's profile.