Lots of great information in this thread and none of it is wrong. My gravel bike is an 80's mountain bike and it does great for all the non-paved road riding I do. That means dirt trails, crushed limestone paths and plenty of gravel farm roads. As somebody mentioned, they're not the best climbers, but for me it just means I can't get out of the saddle and mash the pedals. Spinning up the hills works just fine. Probably not for everyone, though. I don't always need the perfect tool for the job.
I think all the bikes mentioned in this thread would make a fine gravel bike depending on what that means for the OP.
'85 830 current build