Once you have decreased the gusset so that the number of stitches now on your needles is the same number as the number of stitches you started with, you are ready to just knit-knit-knit. Knit each row (no increases, and no decreases).
Measure your foot. Just lay down a tape measure or ruler and stand on it. Put the back of your heel on 0 and see where the longest toe lands. For me that is 9 inches. You will continue to knit until the foot of your sock is 2 inches less than your measured foot length. So for me I knit until my sock measures 7 inches. When you measure your sock you will measure from the back of the heel out. My socks (made to a completed length of 9 inches) will fit women who wear a shoe size from about a size 6 to about a size 9. There is stretch in the socks. This helps when I am making Christmas presents! I don't have to have everyone measure their feet. Here is a general guide for sock sizes, I pulled this from this web site. So if it is a little blurry, please go back to the original site. So if you know a person's shoe size you can knit them a pair of socks.
This is how you measure the foot of your sock. The sock pictured is a finished sock, but you can see it is 7 inches from the heel to where I started decreasing for the toe.