Question about the ETH and DAPPs

Ok, I wish to understand if DAPPs must use ETH to run. For example, can someone create a dapp that is 100% free to use, like open source software is free to use, and the only time that ETH is required is when the code is registered onto the blockchain? (I'm assuming that's how it works).

Or a more specific example, if I wish to code a game and make it completely free, is there a need for ETH at any stage of the process? Must an end-user have ETH in their wallet to get into the game and play it, for example?

edit: just noticed I wrote "the ETH"in title, haha!