Generally I would counsel against using a TWO-wheel trailer on the GDMBR! You will have problems on most single-trail sections. These sections are usually short, but you will hate this type of trailer soon.
For the route between Banff and Whitefish, from memory, the first single trail already occurs some twenty miles south of Banff where you traverse the Spray Lakes Reservoir, but you could omit this section by keeping on the Smith-Dorrien Rd east of the Reservoir. I don't remember single trails between the Kananaskis Lakes and Elkford and between Elkford and Sparwood. If I remember well, there are no single-trails in the Fernie Alternate, but the Flathead route would give you problems, especially the section along the Wigwam River and the connector to Galton Pass.