The gear train is a non-issue in the HG (in my opinion). Very over-built. The HG uses a Clark transaxle and it's pretty much the same unit as Case put a 188 diesel in (310 crawlers).
Also has a 9" clutch (same size as a Model A Ford) and bigger then in my Toyota motorhome.
I've yet to see any trans, differential or final drive parts break from power. Just fail due to lack of lube, worn splines, loose bolts, etc.
The differentials in the HGs and OC3s had a habit of getting loose bolts that hold he differential together.
What I HAVE seen is when people try to push dirt to pull something slowly without the AUX trans - they have to lug the engine something awful at low RPM and/or slip the clutch which is bad for both.
My HG is run in 1st or 2nd AUX gear all the time. When I stick it in 3rd (which is like having no AUX) it's so fast in low gear - it's hard to believe it could push anything and last.