I'm not even sure what a "Gen-X" novel would be. There is of course always the novel which coined the phrase. There is also hugely long crap like the Gravity's Rainbow which you mentioned, which seems to be popular among certain circles, for reasons I do not understand. A similar one in this category would be Infinite Jest, which I could only get about 10-15% of the way through.
I guess I'd have to say the best "Gen-X novel" I've read - if you can call it that - would have to be Snow Crash by Neal Stephenson.
Not that it holds a candle to Huck Finn of course.
It's about codebreakers in WWII, and their computer geek descendents in the mid-1990s. A fantastic book, amazing how similar it is to the vastly overrated Gravity's Rainbow and yet how much better it is. You read it and realize it's essentially screaming in the face of Gen X: "GOD! COUNTRY! CAPITAL! THESE THREE THINGS WILL ALWAYS SAVE CIVILIZATION!"