His Passive (Colossal Smash) actually augments his basic attacks to do AoE Magic Damage - the Magic Damage Shield is a passive effect tied to his W, Shield of Durand - which is a Damage Resist + Charged AoE Taunt, with the shield tied to being out-of-combat and his Justice Punch is a more telegraphed lunge + knock-up, unlike Shen's Shadow Dash and Ki Barrier - the latter of which is tied to ability casts, and the former being both a non-telegraphed dash and line-skillshot Taunt.
The only thing really comparable to Shen is his Hero's Entrance, with it only being able to be targeted on Allied Champions - but the differences continue even there. Galio doesn't follow his ally's movement - he lands where they were when he starts his ult.
So, let's go over this with a different format than just my usual paragraphs of text:
Shen:
- Passive - Ki Barrier: Grants Shen a Physical Damage Shield upon casting an ability, shield cooldown lowered by damaging enemies with abilities.
- Q - Twilight Assault: Pulls Shen's Ki Blade to his position, makes his next three basic attacks do %Health Damage, and if it passes through an enemy Champion, Shen gets an Attack Speed Boost, and the enemy is slowed when moving away from Shen.
- W - Spirit's Refuge: Creates a zone around Shen's Ki Blade which causes all basic-attacks done to allied champions inside the zone to miss.
- E - Shadow Dash: Shen dashes in a direction, damaging and taunting any enemy Champion hit. Also passively allows Shen to regain Energy by damaging enemies with abilities.
- R - Stand United: Shen grants an allied Champion a Physical Damage Shield, and after a brief channel time, teleports to their location, bringing his Ki Blade with him.
Galio:
- Passive - Colossal Smash: Every eight seconds, Galio's next Basic Attack deals AoE Magic Damage - cooldown decreased per unique enemy hit by basic abilities.
- Q - Winds of War: Galio beats his wings, sending two magical winds in narrow arcs ahead of himself, before colliding and creating a tornado that persists shortly afterwards, dealing %Health damage. Both halves of the ability do magic damage.
- W - Shield of Durand: Passively, if Galio remains outside of combat, he gains a Magic Damage Shield. Galio can activate and hold his Shield to decrease damage taken - and when he releases it, performs an AoE Taunt, the size of which increases the longer his defensive stance is held.
- E - Justice Punch: Galio hops back a short distance, before throwing himself forward, colliding with either enemy Champions or Terrain - if he hits an enemy Champion, he performs a powerful uppercut, knocking them up.
- R - Hero's Entrance: Galio targets an allied Champion, grants them the increased damage resistance from Shield of Durand, and leaps high into the air before launching himself at the location that ally was when he began the cast: When he lands, he deals area magic damage and knocks enemies up.
If anything, his kit is only vaguely familiar to Shen's because they have a taunt, an ally-targeted ult, and share the Warden subclass of Tank.
costume is 
cosplaying as a gargoyle.
