You should only pick Galio against an AP mid because of his amazing defensive/offensive passive. You mainly want full lane dominance or else he can be useless. Although he can be good against some AD champs because of his W countering aa based champs. However unless they have an AP jungle or top you will have to build make 0 use of his amazing passive. Galio is mainly a utility champion that impacts teamfights immensely and still does really solid damage.

Use Q as many times as possible in lane because of that extra mana regen from the 2 faerie charms that you started with. Once you get level two you are the bully in lane, use Q followed up by an immediate E for about 125-175 damage early game, that's alot. When you get level 6 don't use it in lane unless your jungler is ganking or you have a guaranteed kill. Most of the time you want to save it to roam.

Galio surprisingly is a roaming champion but not like a normal assassin, he TP's to lanes from base with homegaurds and ultis to get free kills so you want to get homegaurds early on. Tell your laners to ward in lane to give them a extra edge on the game!

Galio is one of the most impactful AP mids in teamfights. You want to wait for the perfect opportunity to engage with a flash ultimate on 3 or more people on their team. His ultimate can be used under towers for some serious turnaround damage. This is strong because of the taunt on his ultimate forcing the tower to attack the first person to attack you for extra damage.