If some of the targets become illegal for Ravenous Gigantotherium's ability, the original division of damage still applies, but the damage that would have been dealt to illegal targets isn't dealt at all.

The creatures targeted by Ravenous Gigantotherium's ability deal damage back to Ravenous Gigantotherium before dying if they're dealt lethal damage.

Ravenous Gigantotherium can't devour creatures entering the battlefield at the same time as it.

You divide the damage as Ravenous Gigantotherium's triggered ability is put onto the stack, not as it resolves. Each target must be assigned at least 1 damage. You can't choose more than X targets and assign 0 damage to a target.

The value of X is locked in as you divide the damage while putting Ravenous Gigantotherium's ability onto the stack. Changing Ravenous Gigantotherium's power after that time won't change how much damage is dealt.

Static abilities, such as that of Glorious Anthem, modify Ravenous Gigantotherium's power before its triggered ability is put onto the stack. Spells, activated abilities, and triggered abilities can't raise its power in time to change the value of X.

You may choose zero targets. If you do, Ravenous Gigantotherium deals no damage and is dealt no damage.