Will Shimano GRX levers work with cantilever brakes? Related question here but answers more about shifters.
If you want mechanical cantilever brakes with a GRX crank and derailleurs you can use mechanical brake road shifters.
Tiagra 4700 series shifters are compatible with with 10 speed GRX
105 and Ultegra shifters are compatible with 11 speed GRX.
You also have the option of using mini v-brakes with road shifters as these match the cable pull of the brake levers in the shifters. Note regular MTB v-brakes should not be used with road brake levers as they use a different cable pull.
No. GRX levers are only designed for hydraulic brakes. All cantilever brakes that I know of are actuated by cables.
Edit: per Adam's comment, Magura does make hydraulic cantilever-style rim brakes, e.g. the HS33. However, these pair to Magura's own flat bar brake levers. I was not aware of this fact, and I didn't consider these brakes.
In Googling the Magura brakes, it does seem like some people used them on mountain bikes in the mid to late 2000s as disc brakes were growing in prevalence but many older MTBs had fittings for V-brakes. Thus, they might have sufficient power for cyclocross. However, to pair them with GRX shifters, I am not certain that the mineral oils that Magura and Shimano use as hydraulic fluid are completely inter-compatible. Mineral oil refers to a general class of hydrocarbons, so the composition and additives might differ between manufacturers. If they are not compatible, you would risk damaging the rubber seals or other components of the brake calipers and/or the shifters. A Magura rep in 2013 swore that the fluid they used then was incompatible with Shimano fittings. I have seen people on forums say they use Shimano oil in Magura units, but I am honestly not sure I would chance it. I'm more writing this out of general interest.