Let (M, g) be a smooth compact Riemannian manifold of dimension n ≥ 2, 1 < p < n and 1 ≤ q < r < p * = np n−p be real parameters. This paper concerns the validity of the optimal Gagliardo-Nirenberg inequalityThis kind of inequality is studied in Chen and Sun (2010) [12] where the authors established its validity when 2 < p < r < p * and (implicitly) τ = 1. Here we solve the case p ≥ r and introduce one more parameter 1 ≤ τ ≤ min{p, 2}. Moreover, we prove the existence of extremal functions for the optimal inequality above.