Couple things you can do
Double the layer and put the top layer into multiply mode
or
assuming the background is white? duplicate the layer and colour to alpha it (making the white transparent then dulicate that layer a few more times.
or
Using threshold should make your grey darker this is actually your best option.
or
using curves you bring the curve down in the centre
or using levels at value bring the centre arrow to the right
Double the layer and put the top layer into multiply mode
or
assuming the background is white? duplicate the layer and colour to alpha it (making the white transparent then dulicate that layer a few more times.
or
Using threshold should make your grey darker this is actually your best option.
or
using curves you bring the curve down in the centre
or using levels at value bring the centre arrow to the right
