MTGNexus

Cards such as Past in Flames give cards in the graveyard Flashback where the Flashback cost is the card's mana cost. How does this work with cards that have no mana cost, such as Gaea's Will?

They would be the same as if you tried to cast them from your hand. That is, they have no Flashback Cost which means there is no cost to pay which means you cannot cast them.

Normally, from your hand, you can use something like As Foretold to give them a cost you can pay. However, that doesn't work with Flashback since Flashback is the cost allowing you to cast from the graveyard and you cannot combine multiple alternate costs. So, generally speaking, Flashback on a spell with no mana cost isn't going to work.

On a side note, based on your title, they still have a Mana Value (MV), just not a mana cost. Their Mana Value is 0. That doesn't change anything but can be worth pointing out.