In a way, it is. The movie takes elements from real cowboy stories and weaves them into the plot. But it also adds some fictional elements for dramatic effect.
In a way, it has roots in real-life situations. However, like many movies, it might have taken some artistic liberties to make the story more engaging and cinematic.
It's not a true story. Usually, movies like Concrete Cowboy take creative liberties and combine different ideas and concepts to form a compelling fictional tale that engages the audience.