Biggest problem I have with a bike like that is down tube shifters don't play well with knobby tires. I've missed a shift and torn up my hands! ;-)
That bike looks like it has a short wheelbase and a steep head tube. Not great for gravel. I wouldn't put any money into it for gravel.
I also find that the rear tire tends to be close to the seat tube and that the chain stays tend to be pretty narrow - making anything bigger than 32mm hard to fit. A 32mm probably won't fit under the brake bridge. Nice frame, but not designed for gravel, and not worth the $$$ to modify, IMHO.