What fills in that gap? The gusset. The gusset consists of forward (or toe-ward) facing stitches. Part one: when you've knit a heel-bottom row, pick up the stitches on those vertical sides of the heel flap,
It helps if you've followed directions to slip a stitch at the beginning of each row, because then there are looser stitches to pick up. If you didn't (I didn't) you get to mutter at yourself as you struggle to pick up the non-loose edge stitches. Those stitches will face forward, and fill the gap (more than fill it, as the foot gets flatter toward the toes) and now all your stitches--bottom, top, and sides--are facing forward. The only tricky bits are the decreases on each side to reduce the too-many-stitches you need at the back of the foot to the number you need for the front of the foot. The only tricky part about that is remembering that some decreases "lean left" and some "lean right" and you want to use one of each--but the same one on the same side of the foot every time. I did that. I also chose to decrease to a little more than my starting number of stitches, because my ankles and bottom of leg are thick (puffy, actually--used to be skinny), but my foot is narrow. My mother's sock had a 48 stitch cast-on--I used 64--but decreased the foot beyond the gusset to 60 in one sock and 58 in the other (test cases--I'll see which I like or if they both needed to be brought down more.)
Past the gusset, it's just foot until you start the toe decreases. How far that is depends on the length and the shape of your foot....and since making one's own socks is an exercise aimed at more comfortable socks...the only rule is "make 'em comfortable." I'm now into the foot section, and this is what the socks look like now:
It's a very cloudy day, and I was unable to get a good enough picture of the gusset structure even with the back door open for more light and the kitchen light on. My feet are propped up on a horse wormer bucket--that we use for storing bird seed--in an attempt to get more light from the open back door. The flash washes out stitch details. Without the undersocks on I use in these pictures, the socks are wonderfully comfortable on my feet, as far as they go. If it weren't for the four double-pointed needles in them, I'd be tempted to leave one on while working on the other.