Taking a shortcut by just calculating the requirements for boiler and cooker adding up to 54 kW, you need about 5 m3 per hour.
As the fire takes only a few percent of the total load, I would personally not include it in the calculation and simply go for a larger size if I came anywhere near the maximum load.
22mm will give you a max equivalent length of about 8 meters, which in your case is not enough, so it must be a least 28 until the first tee, and after that it might still need 28 depending on the route of the pipe.
No need to go larger than 28 as it will give you an equivalent length of more than 20 meters.
The RGI who fitted the pipe, was not required to install the bonding to the gaspipe, and unless he was also a competent electrician, technically not even allowed to do so.
What he should have done is leave you a notice to get it done by a registered electrical installer.