I just reviewed the Canada website and I don't see a lot of GC engines used on snowblowers. The only model that use a GC engine is the HS520. The HS720 uses the GS190, all the 2 stage snowblowers uses GX engines. As you said, the HS621 is the only SS with a GX engine.
The GS series engines appear to be better than the GC series but not quite up to the standards of the GX series. Here's Honda's description:
"Honda GS Series of Overhead Cam Engines
The Honda GS Series is positioned squarely between the GC Series premium residential and the GX Series commercial engines.
The GS190 horizontal shaft and GSV190 vertical shaft models are designed to serve both premium residential and light-duty commercial markets. Intended for use on such popular applications as pumps, blowers, mowers, and tillers, these Honda models satisfy the homeowner seeking ultimate performance, and the contractor seeking a lightweight, compact package.
Capitalizing on the same basic OHC architecture as Honda's extremely popular GC engines, the GS features uniblock construction and a unique internal timing belt design that results in the lightest and most compact package in its class. With 187 cubic centimeters of displacement, this efficient design also delivers an unparalleled power-to-weight ratio with quiet operation, and easy starting.
To further enhance performance and durability, the Honda GS engines incorporate a cast iron cylinder sleeve and a redesigned piston with a three-piece oil control ring. Adding to the engine's commercial character, the GSV190 features ball bearings on the flywheel side of the crankshaft, while the GS190 utilizes ball bearings at both ends of the crankshaft. A dual stage air filter with an oiled pre-filter enhances engine performance in the dusty environment often created by mowing or tilling.
The Honda GS engines exemplify Honda's fundamental design philosophies of durability, quality, reliability, ease of starting and high fuel economy."
Sounds like the GS series are decent engines. Strangely enough, I have a Honda GX series OHC, V-Twin in my 2005 riding mower at camp. Usually the GX series engines have push rod actuated valves. I was surprised to see an OHC engine with the GX designation.
Here's a good link if you want further info concerning Honda's engines:
http://www.honda.com/newsandviews/article.aspx?id=1180-en