Thermally induced valence mixing Fe2+ + Fe3+ ↔ Fe2.5+ is studied in GdBaFe2O5+w by 57Fe Mössbauer spectroscopy. For compositions close to w = 0, a two-step Verwey-type transition is registered that separates Fe2.5+ into intermediate valence- and spin states Fe2.5-ε and Fe 2.5+ε and then into the integer valences Fe2+ and Fe3+. Both steps are accompanied by a proportional decrease in electrical conductivity, altogether by approximately two orders of magnitude, and small magnetoresistance peaks are observed. Seebeck measurements identify holes as charge carriers. Increasing oxygen nonstoichiometry eliminates rapidly the first step, whereas the generally stronger Verwey transition proper persists up to levels of w≈0.25.