A perturbative method to calculate gamma spectra in the hypothesis of an incoherent superposition of all individual nucleon-nucleon contributions is proposed. It is based on the numerical solution of the Vlasov-Uehling-Uhlenbeck equation. The elementary gamma -production processes are assumed to be np to n'p' gamma and the main decay in two gammas of pi ⁰ from the N₁N₂ to N₁'N₂' pi ⁰ reaction. Comparison is made with recent experimental data for a ³⁶Ar+²⁷Al collision at 85 MeV/nucleon.