I was searching GPC for the adventure of Sir Gareth and the Red Knight (sometimes called Castle Perilous). I couldn't find it. Is it in one of the other books?

I don't think it is. The best place to look for it would be Perilous Forest, but I didn't see it in there either, although that book does have some information on the two ladies involved in Gareth's adventure.

This is the best list that I have seen of the adventures that have been published:

Very nice list. Thank you very much!