Vitamin C is just one developing agents in Gainer's formula and other commercial products, and presence of Vitamin C itself does not determine the entire working of the developer. It should be noted that there are developers containing Vitamin C that work very differently.(posted 8158 days ago)In particular, XTOL and Gainer formula are quite different, partly because XTOL sets its pH rather low and well buffered, while Gainer's formula is rather opposite, relatively high pH. I haven't thought about Ilfosol-S much, but I think hydroquinone or other developing agents in the formula may contribute to its working much more than a small amount of sulfite.
For those who like to see a couple other options of Vitamin C formulae, I have a page on matching films and developers that has two Vitamin C formulae, one is like D-76 and XTOL, another not too different from Ilfosol-S.