Great guide! I don't even play Galio, just looked this up right before ranked match and did pretty well with it. Might be my go-to mid laner now, super fun.

Some suggestions (IMHO) to improve this guide :3

1) In the purchase order, separate the items in "Starting Items" , "First Recall" , "Core" , "Situational" (or summarize the last three in a simple "Purchase order"). This makes the guide more understandable and easier for beginners.
2) In "Introduction" explain with some and simple words why do you think a player have to play Galio, why do you think that is strong (do you haven't explained his fantastic passive) and your experience with Galio. Justify the test and add a subtitle named "Lore"
3) A screen of your rune page (maybe with a great border) for Galio can make the guide more aesthetic. I think this is not the place for the passive's explanation.
4) Like the Rune section. A desscription of each masterie or of each section is appreciable ;)
5) Don't write only for the final items. Write about the start, the itemization during the game and what to do if there is an unexpected action. Add the image for each item, and write about it.
6) Add an image of the skil order. After that, explain each skill with its image and write the official description and add what do you have written yet. Move this section before "Item" section.
7) Move the Pro/Cons between "Introduction" and "Runes".
8) The section "Creeping/Jungling" isn't necessary. Write about farming with skills in "Skill Sequence" section.
9) "Team work" section is ok, but write more about teamfights. Ok, we know how to with our allies, but against the enemies? Write about the enemy that you have to pay attention, because some CC stop your ultimate.

last but not least:
Justify all, is very important.

I hope that my suggestion has helped you, because i really like Galio.

Ty for the suggestion i changed the core build a bit and replaced the Banshees veil with Rabadons because you are right with that much liferegen and Mr i got i dont have to be afraid.

And if you have enough gold you can sell your Ga ones its popped and then buy a banshees instead.

Maybe switch banshes veil as no sane ap carry would blow their spells on you when you have that much mr and also guardian angel.
You have so much manaregen that the mana from Banshes won't be nessesary.You could instead switch abyssal scepter.
After all you are the ap carry of the team

Change a point in "Mental Force" to "Juggernaut", those 3% hp and 10% CC-reduction is much strong than 1 ap.
Anyway good guide but I feel that you really should place in the rabadons as it's criticall for your damage.