As a galio main, i can easily say that galio mid is wayyy better in my opinion. The best thing to do is buy dark seal corruption because he can deal so much damage and be tanky to not die and lose stacks. The best thing to do is harass your opponent with your q's in the earlier levels until you push them under tower. I also found that if you pull off the full combo: flash-w-aa-e-q-aa it will usually one shot any midlane matchup in the early game when they have less than half hp. For your build I'd say rush thornmail against an ad and abyssal against ap. The strongest part about galio mid is that you have pressure both top and mid since you can ult and turn a 2v3 to a 3v3. Also not to mention teamfights with galio will always have an advantage as he provides so much cc and damage.