OT - how my yogurt came out

I'd suggest mixing the yogurt into some of the milk instead of shaking the jars then mixing it back in, gently. You might also let it go longer. I let mine sit from 12 - 24 hours.
Also, don't stir your yogurt until you're ready to eat it. Stirring tends to beak down homemade yogurt.
Gelatin won't make a difference except in texture. If you add it you get a more consistent texture instead of a creamy layer at the top.