While there is no permanent "free" version of the full game, you can access it legally through these methods:

: The game has previously been offered as a free "keep forever" title for Prime Gaming members. Check your Prime account to see if it is currently available or in your library.

: It has been included in the Xbox Game Pass library for PC and console, allowing you to play at no additional cost if you have an active subscription. Where to Buy Ghost Song

If you prefer to own it, the game is available on major digital storefronts: Go to product viewer dialog for this item. Ghost Song