Anyone that is making curved top tube frames with a constant curve is using a tubing roller. The most common one one is this Harbor Freight roller
https://www.harborfreight.com/tubing...psugg_q=roller with dies from Swag Offroad
https://www.swagoffroad.com/SWAG-Tub...ries_c_54.html
Rolling a tube requires the tube to have sacrificial material on each end, because you cannot roll the tube all of the way through the full length of the tube. You will have a flat section on each end of the tube that has to be cut off, so a long straight gauge tube is what is generally used.