Using spectroscopic ellipsometry measurements on GaP 1−x Bi x /GaP epitaxial layers up to x = 3.7% we observe a giant bowing of the direct band gap (EgΓ) and valence band spin-orbit splitting energy (Δ SO ). EgΓ (Δ SO ) 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 Δ SO in going from GaP to GaP 0.99 Bi 0.01 . The evolution of EgΓ and Δ SO 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 EgΓ and Δ SO with x. In contrast to the well-studied GaAs 1−x Bi x alloy , in GaP 1−x Bi x 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 EgΓ and Δ SO 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 GaP 1−x Bi x alloy band structure.