Hi Christian,
at least currently this is not the case. Technically, there is one important difference that makes GPU application much more difficult (and less efficient) for FEFLOW: FEFLOW uses an implicit method, while M21 is based on an explicit formulation. The latter allows to parallely solve the equations for the single nodes, leading to the high gain factor when using GPU. We should not forget, however, that this comes with a higher price in terms of runtime beforehand, as explicit methods require a much shorter time step.
Best regards,
Peter