Major capsid protein that self-associates to form 160 hexamers, building most of the T=13 laevo capsid in association with 11 pentons made of gp24 and one dodecamer of gp20. Folding of gp23 requires the assistance of two chaperones, the host chaperone groL acting with the phage encoded gp23-specific chaperone, gp31. The capsid also contains two nonessential outer capsid proteins, Hoc and Soc, which decorate the capsid surface. Through binding to adjacent gp23 subunits, Soc reinforces the capsid structure.