Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Galio is a good all round mid laner because he has one of the best teamfight ults (tho it can be countered), has great wave clear, has pretty good sustain, and is very, very hard to tower dive. I personal am not a mid lane main but when I am forced into mid lane its usually with galio for with this build you can become very tanky within the first 2 items being completed, he also has some very nice base damages to get you to the point of getting the deathcap and void staff, and can get some very good roams down to bot lane post level 6. With the boots and masteries it gives you the most possible flash + ults you can get. This being said with this build its hard to secure kills and you have to be a teamplayer (and who likes to do that) and also is very mana hungry early game but later in the game you probably won't even need blue to keep you mana up. Since Galio is one of the lest played champion in league of legends you can throw off all but most experienced mid laners.