My comment earlier about re-gluing the gasket was based on some emails to Weber customer service about how to reattach a screw/bolt thing that fell off the lid of my WSK. Since I last posted, I went and looked at my lid again, and I now suspect glue might be unnecessary.
I was advised that I had to remove the gasket with a putty knife to open up the lid to realign everything to get the screw reattached properly. Customer service instructed me to glue the gasket back in place with high heat silicon glue/lubricant such as Permatex high heat RTV or similar.
I haven’t gotten around to it due to it being our busy season at work, among other things, and because there’s no noticeable leak out from where the screw fell off, I’ve just continued cooking. Also, opening this lid seems like an extremely involved process, and I’ve had other things to do every weekend. 
Out of curiosity, I just went and gently pulled on my gasket, and it appears to be held in place with thin wire spring clip things. I don’t see any sign of glue having been used. I’m pretty sure I can reuse the wire spring clip things, so we’ll see how that goes.
Weber customer service did offer a replacement gasket if necessary, and a replacement lid assembly if the attempted repair proves unsuccessful.