Ok, so I had that problem, too! Then, I learned that you should pick up one or two extra stitches and decrease them in the decreasing rounds. This solves the problem of holes in the corner.
When you knit socks, do you tend to get holes along the rest of the gusset where you have picked up stitches? Well i do, and i learned from DIY that you should knit the stitches that you have picked up through the back of the loop the round after you picked them up. This twists them and makes them tighter!! Hope this information helps!!