It looks like the 2g, 3g, 5g refers to the generation of the manufacturer's product. For example, J&J makes Color Splash 2g and Color Splash 3g bulbs. The Color Splash 3g has some technical advancements over the Color Splash 2g (and is pricier), but the comparison of "g" doesn't make sense when you go across brands. In other words, one brand's "3g" may be the same as another's "5g". I only see J&J and Pentair using these kinds of terms for their bulbs.