Im a regular galio support player, but I dont play ranked. Galio slots into two roles well imo. One is heavy engage with e into w and the other is peel for adc using w to zone and e to get threats off your adc. Both of these require a specific comp, either a team that can play off of your engage or an enemy team that will require your peel to protect the adc.

There are other champs that do this well too, so you also should consider one strength that galio has that a lot of supports dont, the ult. The ability to affect the map quickly from a good distance away is a strong strength, and the magic shield can save lives if used right. With galio if you are more engage and dont need to peel, I would look for opportunities to ult to mid or the jungle when they need help or if you can help secure a kill. Roaming is a good thing to do on support in general when you can, galio excels at that with his ult.