For a first frame, I would use Nova Cycles house brand tubes. I've used their tubes for many frames and never had any problem with them. For gravel frames, I use a 34.9mm 9/6/9 down tube, 31.8 8/5/8 top tube and an external butted seat tube for a 27.2 seat post. I use Paragon 44mm head tubes, because you can run any steerer size by changing the headset and they are thick enough for a beginner to deform them and still ream it back to size.
I use straight walled .035" 4130 tubes for chain and seat stays. I use 3/4" for chain stays and 1/2" for seat stays on gravel frames (MTB frames get 5/8" seat stays). You would be better off using pre bent stays for the first few frames