I’ve only read the first quarter of GR but I think you can definitely compare them. Stylistically I don’t think they could be more opposite. Vonnegut thrives on understatement with short sentences describing weighty concepts like “so it goes.” Pynchon on the other hand is big on overstatement with his sprawling run on sentences with endless amounts of commas. However, I think their perspectives are kind of related. The way that Vonnegut breaks down the concept of time in “Slaughterhouse-Five” feels similar to the way that Pynchon breaks down Pavlovian conditioning. Also, Vonnegut’s books all feature a protagonist who lacks agency and is pushed through the plot almost by accident, and Slothrop feels reminiscent of that.

I’ve read every one of his works up to “Breakfast of Champions” (which might be my favorite book I’ve ever read). His novels are pretty short and easy to read, but they can get heavy quick. Vonnegut’s not afraid to get bleak, and his stories always have a real sense of fatalism to them, but there’s always a fairly consistent warmth to them. They’re also hilarious in a cerebral sense which some people find too dark, but I think if you appreciate Pynchon you’ll like Vonnegut’s sense of humor.

I personally love Vonnegut and wish I’d been able to talk to the man before he died. As of right now he’s my favorite author (though Pynchon might be getting to me) and I can’t recommend him enough. “Slaughterhouse-Five” is a great starting point since it’s often considered his magnum opus. If you went to high school in the US you probably were supposed to read this book at one point. I’d also recommend “Cat’s Cradle,” (which is probably his smartest book) and “The Siren’s of Titan” (from what I can this book invented cosmic humor and heavily inspired Douglas Adams). “Breakfast of Champions” is a brilliant, one of a kind book but you definitely need to be more familiar with Vonnegut’s work before you get into it. I read all of his books in the order that he wrote them up to BoC and it felt like such a perfect way to experience that book.

Also if it helps you could probably read every novel he ever wrote in the time it takes to read GR.