"and says i can use any size to obtain gauge"
What this means is that the most important thing is the GAUGE not the needle size. Lets say the pattern calls for a size 7 needle to get 5 stitches per inch, but you switch out the yarn and this new yarn calls for a size 8 needle to get 5 stitches per inch. Then you would use the size 8 needle, because the most important part is to get 5 stitches per inch.
You could for example, take a sport weight yarn, use it with much larger needles and get it to have 5 stitches per inch. Not sure if you would like how it looked (hey, you could love it!) but the point is gauge is the key. So if the pattern called for size 7 needle for 5 stitches per inch, but you have to use size 8 to get 5 stitches per inch it would look like this:
Pattern calls for size 7 & 6 needles.
You would instead use size 8 & 7 needles. Both go up by 1.
I agree with ingrid. The smaller needle could be used for several reasons. For example garter stitch swatch knit on the same size needle as a stockinette stitch swatch, the garter swatch will be WIDER and shorter in most cases! The stitches change so does the size. A smaller needle is often used on borders for a neater and tighter appearance.