What would be the best bike frame to build a gravel bike?
When I was considering building a gravel bike from an older frame I considered two basic frame types: cyclocross and drop bar commuter.  A light touring bike frame would be a good basis as well, although many older touring frames were built for stronger 26" wheels.

The primary features you are looking for are cantilever brakes and clearance for at least 32mm preferably 35mm or more tires. Another is possibly the ability to mount fenders, which you will find on a commuter or tourer.

Steel seems to be the material of choice for durability and comfort, but if you value light weight Al can be made to work. It really depends on what compromise you want to make.

In terms of geometry take your pick between more aggressive cyclocross or more relaxed commuter. One thing to bear in mind is older cyclocross bikes predominantly have a 'Euro-style' high bottom bracket.