GaN transistors attract much interest due to their low conduction and switching losses. However, their fast switching and low gate threshold voltage make them prone to gate instability issues. Furthermore, they suffer from current collapse issue that is notably known to impair on-state resistance. This paper focuses on commutation behaviour and shows how current collapse also impacts the circuit stability by altering immunity to gate oscillations. It is shown that the conventional double-pulse test (DPT) may not provide reliable assessment of the switching cell design. Theoretical analysis is carried out and a modified DPT test bench is proposed to evaluate the influence of current collapse on GaN converter circuit stability. The analysis is validated by experimental results based on the modified DPT setup.