The shrine's rather short duration of 2 minutes and the long term oriented idea of gf/mf don't really mix well. It's obvious that the 25% bonus to gf/mf for 2 minutes isn't going to be very appealing when you could be blasting through mobs with other "buff" shrines.
Maybe you could change this shrine so that it gives an overall +% bonus instead of a flat percentage boost? Or maybe make it so that some types of items drop in higher quantities? Making gems drop at higher grades is also interesting.
The shrine system is great, but I think the gf/mf shrine could be a little more useful.