I know I knit top-down socks with a gap solution other than the usual pick-up-stitches-below one, but I did not quite understand what I was doing nor did I take very good notes. So I knit up a quick sample sock in worsted weight yarn, to try to figure things out.
I think the most important step is to start heel flap on the wrong side, slipping the first stitch purlwise. This makes the first stitch a continuation of the last round, which in turn prevents the creation of an extra row.
For this sample sock, I slipped a stitch purlwise with the yarn in front and purled the WS rows of the heel flap, and (sl1 purlwise with the yarn in back, k1) repeat across the RS.
When the heel flap was long enough, I ended with the WS.
When working the heel turn, I STOPPED before last purl row. I've always had trouble getting the same number of gusset stitches on each side of the sock. I think not working the final back-and-forth on the heel turn helps prevent this. I'm not sure it is necessary regarding the sock gap, though.
Then I picked up gusset stitches, worked across the instep, picked up the same number of gusset stitches on other side of sock. Crossing the heel turn, I K2tog the remaining "gap" there before continuing across the heel.
Finally, I worked the gusset as I would normally, decreasing one stitch on each side every other round.
Depending on your sock pattern, you may have to adjust these instructions. Remember: the important thing is to not create an extra row.
My current pair of socks on the needles are toe-up, but I'll knit the following pair top-down and update this post if necessary. Again, if you have questions, please leave a comment and I will try to make this more clear. Hope this helps!