I've crocheted them into durable rag rugs. Instead of knotting the pieces together, I lay the end pieces one overlapping the other and take a quick stitch, either by hand or machine. But I saw a tutorial on youtube that showed how to put a slit in each end of the pieces and join them, making a flat join. I've not tried it yet ... just have not had enough t-shirts.
Your GD's quilt turned out beautiful!