I choose Galio top over Maokai because he more directly supports his friends with buffs, while Maokai is more enemy-oriented with a shorter-cooldown hard CC, and only one buff. Yet I consider them to be very similar.
Galio's Level 3 damage is surprising, even his auto-attack nets me a surprising number of kills. Resolute Smite is a soft peel, and stronger when combined with Righteous Gust. Both the utility and the damage of Resolute Smite improve with levels, and enemies are not always big MR builders. To capitalize early game, I would recommend levelling Q before the enemy gets his second Null Magic Mantle. Then W, not E. Because the enemy will start doing some damage, and your pal needs the extra armor while they run from the guy you slowed.
Galio's ult synergizes well with Odyn's Veil. Catalyst is a viable alternative to Chalice, especially with the speed of experience gain on the Crystal Scar. Mercury's Treads are a good choice against any team, but on Galio it is better, so I rarely have to think about boots; if I'm doing super-well I might get Sorcerer's Shoes but rarely. For armor it is usually Frozen Heart because of the CDR and more mana; often I buy at least Glacial Shroud while sitting on Catalyst, instead of trying to finish Odyn's and having no armor for too long. Once I have Odyn's Veil for the health, I can't always buy Abyssal Sceptre. The most important thing on a support-tank is CDR, and I top it off with a Kindlegem item before fitting Abyssal or Void in. Try Hextech Sweeper.
People complain that tanks don't do damage, but CDR is a damage source. And you don't need to do damage late. You do some damage early, and lategame you need to turtle and bodyguard. Anyways, if you can't get into Galio, try Maokai.
(Edit: My real name is Vicki Nemeth.)