Ghalta the Unstoppable's cost reduction ability can't reduce the total cost to cast the spell below {G}.

Ghalta the Unstoppable's mana value doesn't change no matter what the greatest power of creatures you control is.

Once you announce you're casting a spell, no player may take actions until the spell has been paid for. Notably, opponents can't try to reduce the power of creatures you control.

Once you determine the cost to cast Ghalta the Unstoppable, you may activate mana abilities to pay that cost. If the greatest power of creatures you control changes while activating mana abilities, the cost to cast Ghalta remains what you previously determined.

The first step of casting a spell is to move it to the stack. If this causes the greatest power of creatures you control to change (perhaps because you control a creature whose power is determined by the number of cards in your hand), that new power will be used to determine the cost reduction.