Who said to die is to gain?

To die is gain in a very broad sense, for it is an exchange of hampering conditions for a life without limitation. 

Death is merely the transportation of a peasant to a palace, the environment of which gives him opportunities he never dreamed of. 

We shed bitter tears at a grave, but there is more or less selfishness in our grief. 

If we had full faith in the future, the muffled sound of sighs would be followed by a solemn conviction that while we are somewhat the worse off by what we call bereavement, the departed loved one is much the better off.

The Encyclopaedia of Death and Life in the Spirit World [Opinions and Experiences from Eminent Sources