I'm just learning myself, but I think the Giant gene is similar, and possibly co-dominant, just like the Mack Snow gene.. Two Giants can produce a Super Giant, and two Super Giants (I think) produce 100% super giants.. the Giant gene can be seen in offspring with just one parent carrying the gene..that's about all I know
From my experience with it, I don't believe it's co-dominant, but it behaves similarly to a co-dom. With one giant parent, you'll produce some larger than normal offspring, but you need to breed giant/giant or super giant/super giant to get the honkers