The film was released by Gunpowder & Sky and is currently streaming on Hulu with premium access. It is also available to rent from Vudu and Amazon Prime Video for $1.99, or Apple TV and Google Play Movies & TV for $3.99.
You do, unfortunately, have to pay to rent The Great Wall. And there isn't a cheaper option either. Vudu, Amazon Prime Video, Apple TV, YouTube, and more all have the movie available for rent for $3.99.