In this paper we define and study a filtration (G_s) on the algebra of values at algebraic points of analytic continuations of G-functions: G_s is the set of values at algebraic points in the disk of convergence of all G-functions sum_n a_n z^n for which there exist some positive integers b and c such that d_{s b}^n c^{n+1} a_n is an algebraic integer for any n, where d_n = lcm(1, 2,. .. , n). We study the situation at the boundary of the disk of convergence, and using transfer results from analysis of singularities we deduce that constants in G_s appear in the asymptotic expansion of such a sequence (a_n).