My own support frame is made of 50x50x3 which seems to be fine, so in principle your round tube should be OK.

However, the frame doesn't just keep the machine at the right height, it also has to withstand surprisingly high loads as the gantry in particular accelerates and slows down. Corner joints need to be strong, with plenty of triangulation. Round tube is more difficult to join to achieve this. And please remember that you need to remove zinc from galvanised areas before welding as the fumes are toxic (although maybe you know this already).