Sew the cast-on and bind-off edges of garter stitch together invisibly. This is the same as an invisible horizontal seam on stockinette (which you can see here) but it's a bit trickier to see where to stick the needle in garter stitch.

This technique is used on Color by Number projects if you choose to sew them closed instead of using buttons.

you need the cast-on and bind-off edges, and a blunt yarn needle

hold the two edges together (in the pictures, the cast-on edge is on top, bind-off edge on bottom), and bring needle through cast-on stitch as you see here, under the stitch on the right side of the edge

bring the needle through bind-off stitch, going in the right side...

...then out the next bind-off stitch...

...which you can do all in one step, so it looks like this

repeat, connecting edge to edge, not pulling too tightly, so that the seaming blends with the garter stitch fabric