[en] Superconducting properties of GdBa₂Cu₃O_6+δ monocrystals with tetragonal modification and various oxygen content (δ = 0.2, 0.4, 0.56 and 0.75) are experimentally studied. It is shown that with decrease of oxygen content, transition from superconductor three-dimensional behaviour to quasi-two-dimensional and further to strongly two-dimensional one is observed. It is stated that superconducting transition in samples with quasi-two-dimensional behaviour has a character of Berezinsky-Kosterlits-Tauless transition. It is obtained that fluctations of order parameter in the vicinity of superconducting transition temperature change dimensions from 3D to 2D provided oxygen index decrease