Self-striping and variegated yarns are going to self-stripe and variegate no matter what gauge you get, but that doesn't mean that gauge doesn't matter. Gauge does matter. What it means when it says that it can be between two different numbers is that you can use different needle sizes, get between those two gauges, and still have a nice fabric. For example, if it said that the suggested gauge was between 4 and 5 stitches to the inch, you could get 4 stitches to the inch, 5 stitches to the inch and anything in between, such as 4.5 stitches to the inch.
I hope that helps!