Look at this picture, see how the bottom of it is formed, that is what looks to be cut off
https://www.heartbeatcitycamaro.com/...2-%26-7719483/

 

Here is a good picture
You can click on the picture and zoom in on it. I does look like your gasket got the bottom cut off.
The link is from Detroit Speed

 

The part definitely looks different from what's on my car, and thankfully it isn't outrageously expensive. I'm going to order it and see what difference it makes. Given that the existing gasket is pretty cracked it probably is a good idea to replace it anyway.

I'm not sure I would've guessed the name for that, even though it seems rather obvious in retrospect [(Roofrail Weatherstrip)]. I was able to find removal/replacement instructions in the Fisher Body Service Manual once I knew what to look for.

Thanks for the help everyone.

[Edited to add part name for posterity.]