Officially, 30mm is the max width for this 2by bike. However, I successfully put that 40 mm gravel tires on it today. It has plenty of space on the front fork.
On the back, there is ~ a 1mm space between the tire and fender nut and ~ 2mm on each side between the tire and frame.
The bike is now more comfortable and stable on gravel and wet roads than with the original G-One Speed 30mm. According to the Schwalbe graph, those tires will lose a bit on tarmac but gain a lot on Gravel, dry, and wet trails, and I can confirm that.
by jaco2201