Using spectroscopic ellipsometry measurements on GaP1-xBix/GaP epitaxial layers up to x = 3.7% we observe a giant bowing of the direct band gap ([Formula: see text]) and valence band spin-orbit splitting energy (ΔSO). [Formula: see text] (ΔSO) is measured to decrease (increase) by approximately 200 meV (240 meV) with the incorporation of 1% Bi, corresponding to a greater than fourfold increase in ΔSO in going from GaP to GaP0.99Bi0.01. The evolution of [Formula: see text] and ΔSO with x is characterised by strong, composition-dependent bowing. We demonstrate that a simple valence band-anticrossing model, parametrised directly from atomistic supercell calculations, quantitatively describes the measured evolution of [Formula: see text] and ΔSO with x. In contrast to the well-studied GaAs1-xBix alloy, in GaP1-xBix substitutional Bi creates localised impurity states lying energetically within the GaP host matrix band gap. This leads to the emergence of an optically active band of Bi-hybridised states, accounting for the overall large bowing of [Formula: see text] and ΔSO and in particular for the giant bowing observed for x ≲ 1%. Our analysis provides insight into the action of Bi as an isovalent impurity, and constitutes the first detailed experimental and theoretical analysis of the GaP1-xBix alloy band structure.