Unfortunately, from measuring the track gauge on Trumpeter #00213, "German Railway Track", it's neither an accurate 1/35 standard gauge (1435mm -> 41mm), nor is it a match to the 45mm track gauge that both G gauge and 1 Gauge use; it looks to measure out about 42-43mm (there's a small amount of slop in the fit), which places it somewhat closer to Russian 1520mm (~43.5mm in scale).
Not having started construction on it, I don't know if there is or how much gauge adjustment you can make on the axles of the Trumpeter #0021 "Schwerer Plattformwagen Type SSyms 80", but the instruction for Tank Workshop's resin kit of the same flatcar specifically describe using a piece of track to adjust the position of the wheels on the axles to the correct gauge before gluing them in position.
1 Gauge/G gauge track for scale model railroading appears to be relatively expensive, which is annoying; the Dragon and Trumpeter track is all straight; for the diorama I have planned, being able to have a switch off to a siding that the flatcars were sitting on would add visual interest to that corner of the diorama, but two or three hundred dollars for a casual detail is excessive -- and the gauge mismatch means I'd have to use all G gauge track instead of the Trumpeter track.
Now, without having a section to measure, I can't say what the actual gauge is I can't say how useful it might be, but I found this item at Amazon (or at hammondtoy.com, which actually services the item on Amazon -- search on 'Newqida Toy Factory') -- G gauge plastic track for battery-driven train sets. At $25 for two switches, 16 curved pieces, and four straight, if the track gauge is compatible with the flatcar kits and can be painted and ballasted to hide its origin as cheap plastic track, it makes a viable alternative to metal track (G gauge and 1 Gauge are sometimes referred to as 'garden' scales, because they're popular choices for garden railways, which is one of the reason it's more expensive, as it has to stand up to exposure to the weather).