The god can attack the turn it becomes a creature, as long as you had it for more than 1 turn like you said. Summoning sickness only affects a creature if it entered the battlefield since the beginning of your turn, and it did not enter the battlefield upon meeting its devotion requirement, it simply changed to a creature. This won't trigger "when enters the battlefield" triggers either.
302.6. A creature’s activated ability with the tap symbol or the untap symbol in its activation cost can’t be activated unless the creature has been under its controller’s control continuously since his or her most recent turn began. A creature can’t attack unless it has been under its controller’s control continuously since his or her most recent turn began. This rule is informally called the “summoning sickness” rule.
The god creature HAS been under your control since the beginning of your turn; it just wasn't a creature before.