When joining in the round, this is what I do for that first stitch. I knit the first stitch with both the working yarn and the tail. I drop the tail and finish the round as usual with the working yarn. When I begin round 2, I have a double stitch now for that first one. I knit the two of them (working yarn and tail yarn) stitches together. Pull the tail tight. This closes up the gap. Pull the tail tight again for rounds three and four. After that, you don't have to. Continue knitting around and around and you will have no gap. To avoid ladders when switching needles, move the first stitch as close to the tip of the needle as you can get without it's falling off. Then knit as usual. The needle tips are tapered. Knitting that first stitch near the tip makes the stitch smaller and tightens it. Pull the yarn as tightly as you can while making that first stitch or two, then relax and finish the stitches on the left needle as normal. This works for both double pointed and circular needles using magic loop.
Another thing no one ever told me. I had to figure this out for myself because none of the magic loop or double point instruction videos tell you this. Always be sure your working yarn is coming from inside the tube or BETWEEN the two needles, not from behind the back one. If you place the working yarn behind the back needle, you will get an unwanted yarn over at the end. This will give you an unsightly huge gap where the work joins. Always be sure the yarn is placed BETWEEN the two needles when switching from one needle to the next.
Welsh corgis are cool.