Although she is not known to have had any children of her own, her interest in them was lifelong, exceptionally sympathetic, and surprisingly reciprocal. But they entered her art only in 1976, in the form of an anomalous project, the film Gabriel, which features a young boy.
—Nancy Princenthal on the book Agnes Martin: Her Life and Art
All the film stills above were taken from Tate Modern's website, specifically this announcement of a screening of Gabriel in 2015: