I was working in the engine bay when I noticed that the left headlight gasket came off, just like so many ppl experienced.

I used silicone, Permatex Ultra Black Gasket Maker #598 B, to put the seal back on. It can be bought at your local AutoZone. Seems very strong and most importantly, waterproof. May be better than double-sided tape.

You just squeeze the silicone between headlight and the gasket, wipe of the excess, and tape down the gasket to let the silicone cure.