When you k2tog, you decrease one stitch. You'll still have one stitch now on the right needle. That's why you'll need to bind off 2sts when you do the k2tog.
So you might knit one, k2tog, and then pass st one over the stitch left from the k2tog. That's 2sts decreased. Now knit one more st from the left needle and pass the first st on the right needle over the st just knit. That's 3sts decreased. Ther won't be a gap.