Photoluminescence (PL) spectroscopy is a powerful tool in studying semiconductor properties and identifying point defects. Gallium nitride (GaN) is a remarkable semiconductor material for its use in a new generation of bright white LEDs, blue lasers, and high-power electronics. In this Tutorial, we present details of PL experiments and discuss possible sources of mistakes. A brief analysis of near-band-edge emission includes basic characterization of GaN, essential findings about excitons in this material, and the explanation of less known details. We review modern approaches of quantitative analysis of PL from point defects in GaN. The updated classification of defects in undoped GaN and their latest identifications are presented. Typical mistakes in the interpretation of PL spectra from GaN are discussed, and myths about PL are refuted.