Galio should lean on the tanky side and do reasonable damage if he builds for it, a true AP bruiser. He's sadly left in an increidbly shitty state where if you don't build full tank and become cc bot, he's useless.
Ironically enough, all the nerfs have not affected his support status and he's as a strong as ever in the role. Stay with me here; his poke is still strong on Q, his taunt and dash are cc staples, and you can easily build him full tank and go where you please.
His ult is still pretty useless other than for disrupting enemy team fights, but you can easily add the shield back on with
when you land and have some clutch saves. Couple all that with Aftershock, which is balanced for pure tanks now, and he hasn't been affected by the nerfs in a support role.
The absolute best two items on him currently are
-or-
and
. Tied with the hp and wards from
/
and whatever defensive boots you get early on, plus all that CDR makes for one spammy, unkillable, support that has map control and ultimate team fight control with taunt. His Q, as it stands, also does some %hp damage which can finish off most squishies and wittle down tankier champs alike. That core item build covers all bases and leaves room for any situational item.
He counters every match up too.
Facing lots of sustain?
/
synergizes with your taunt and applies Grievous Wounds
Getting CC'd too much?
not a single bad stat for Galio coupled with 
Facing some tough burst damage?
or
The former adds to your passive a bit and the only questionable stat is the AD, which will buff your passive a bit so it's not totally wasted, and the latter buffs the whole team!
Enemy Team on the defensive and you're team is ahead?
/
/
/
/
provides damage and HP vs all sorts of champions and provide secondary buffs/debuffs that are helpful even if enemy builds some MR to mitigate you.
Need some split pushing action and can't clear waves with just Q anymore?
gets the job done believe it or not without sacrificing your tank stats.
Just get that core of
/
,
-or-
and
and your relevant boots and you can pretty much go any route from there and not ever be useless.