1975 Peugeot PX50L gravel bike.
”Gravel specific” this and that is mostly marketing, and fashion items always command higher prices. Therefore, riding or adapting what you already have is sometimes the most sensible approach, especially when DIY is half of the hobby and if there happens to be boxes of stuff readily available.
Old bikes don’t become new even with bucket full of new components and a welding torch, but can’t really beat the feeling of riding your own creation. Pug above took me about a year and ~1k to build, it’s bit noodle when loaded but it sure is the most versatile bike I own.
For starting points to DIY, I’m repeating what others have already said: old sport tourers are often the best candidates.