  Given any non-polynomial $G$-function $F(z)=\sum\_{k=0}^\infty A\_k z^k$ of
radius of convergence $R$, we consider the $G$-functions
$F\_n^{[s]}(z)=\sum\_{k=0}^\infty \frac{A\_k}{(k+n)^s}z^k$ for any integers
$s\geq 0$ and $n\geq 1$. For any fixed algebraic number $\alpha$ such that $0
\textless{} \vert \alpha \vert \textless{} R$ and any number field $\mathbb{K}$
containing $\alpha$ and the $A\_k$'s, we define $\Phi\_{\alpha, S}$ as the
$\mathbb{K}$-vector space generated by the values $F\_n^{[s]}(\alpha)$, $n\ge
1$ and $0\leq s\leq S$. We prove that $u\_{\mathbb{K},F}\log(S)\leq
\dim\_{\mathbb{K}}(\Phi\_{\alpha, S})\leq v\_F S$ for any $S$, with effective
constants $u\_{\mathbb{K},F}\textgreater{}0$ and $v\_F\textgreater{}0$, and
that the family $(F\_n^{[s]}(\alpha))\_{1\le n \le v\_F, s \ge 0}$ contains
infinitely many irrational numbers. This theorem applies in particular when $F$
is an hypergeometric series with rational parameters or a multiple
polylogarithm, and it encompasses a previous result by the second author and
Marcovecchio in the case of polylogarithms. The proof relies on an explicit
construction of Pad\'e-type approximants. It makes use of results of Andr\'e,
Chudnovsky and Katz on $G$-operators, of a new linear independence criterion
\`a la Nesterenko over number fields, of singularity analysis as well as of the
saddle point method.
