A series of trigonal planar N-, O-, and S-dopant atoms incorporated along the convex protrusion lining the edges of bottom-up synthesized chevron graphene nanoribbons (cGRs) induce a characteristic shift in the energy of conduction and valence band edge states along with a significant reduction of the band gap of up to 0.3 eV per dopant atom per monomer. A combination of scanning probe spectroscopy and density functional theory calculations reveals that the direction and the magnitude of charge transfer between the dopant atoms and the cGNR backbone are dominated by inductive effects and follow the expected trend in electronegativity. The introduction of heteroatom dopants with trigonal planar geometry ensures an efficient overlap of a p-orbital lone-pair centered on the dopant atom with the extended pi-system of the cGNR backbone effectively extending the conjugation length. Our work demonstrates a widely tunable method for band gap engineering of graphene nanostructures for advanced electronic applications.