Top down knitting projects are some of my favorites. They are pretty much seamless except for a small area where the sleeve stitches are picked up on the underside. Picking up and joining the sleeves can sometimes leaves a pesky gap. When I get that gap, here's the easy way I found to close it.
1. When re-joining the yarn to work the sleeve stitches, leave a longer tail than usual (approximately 6" or so).
2. Use that tail to work a duplicate stitch or two to close that gap , then fasten off.