A series of molecular signals initiated by the binding of the cytokine granulocyte macrophage colony-stimulating factor (GM-CSF) to a receptor on the surface of a cell, and ending with regulation of a downstream cellular process, e.g. transcription. GM-CSF binds to a heterodimer receptor (CSF2R) consisting of an alpha ligand-binding subunit, and a common beta subunit that is shared with other cytokine receptors.