Might be easier to decide how much natural armor it should get, and work from there.
Large giants have +8 or +9 natural armor (with the exception of the Stone giant: +11)
The Huge Cloud and Storm giants have +12 natural, but "Cloud" and "Storm" doesn't exactly inspire thoughts of natural armor.
The Colossal Mountain Giant has a whopping +29 natural.
Scaling up from +9 natural for a Large giant would give us +16 for a Gargantuan giant. Scaling down from the Mountain Giant would net +24 natural.
[Edit: BLAH... brainfart!]
With a -4 size and -2 Dex, this would give us a range of 20 to 28 for total AC. Since I prefer a grass-covered ("piebald") rather than a rocky hill, I would prefer to stay on the low side of that range, maybe something like 22-24 AC (+18 to +20 natural AC). That would also fit better with its "soft target" role.
My preference would be to restart this poll with some lower numbers included. If I *had* to pick between the ones listed here, I'd go with 26.