The recently introduced Flamelet Generated Manifold (FGM) method has proved to be an accurate and efficient reduction method for the modelling of premixed flames. The FGM method uses a chemical library based on one-dimensional unstrained premixed flames to model the chemistry of a multi-dimensional flame. Recently, the method has also been applied successfully to a so-called triple flame configuration, which is partially premixed. In this configuration, the gradient of the mixture fraction was relatively small compared to the flame thickness. In this paper the applicability of FGM in partially premixed combustion systems is investigated further. The method is tested in a planar counterflow configuration, which enables the control of the mixture fraction gradient. The gradient of the mixture fraction is changed by modifying the applied strain in one case and the inlet mixture fraction in the other case. The results show that, even though the mixing and flow time scales are of the same order as the flame time scales from the database. FGM is still relatively accurate. This can be explained by the fact that in a large part of the flame, the chemistry is not far from equilibrium, which means that the chemistry is still much faster than the flow and mixing processes. It has already been shown that this holds for strain, but this paper shows that it is also true for the dissipation rate. For very high strain and dissipation rates (up to 2000 s-1), errors up to 10% are obtained.