Statistical analysis on Milky Way globular clusters distribution allows to get the critical value of the onset of gravothermal catastrophe, connected with the maximum of the distribution function. This result, showing that such instability onsets earlier than commonly believed, is confirmed by theoretical calculations taking into account the presence of the effective potential which describes the effects of the tidal forces induced by the hosting galaxy. N-body simulations also confirm the presence of the effective potential with the predicted analytical behavior. With the observational data by the third release of Gaia mission we are now able to recognize the presence of the effective potential in real clusters. Here we present the results for NGC6121 which confirm the theoretical prediction even if in preliminary form.