Jamieson, Fausset and Brown on Philippians 1:21

21. For--in either event (Php 1:20) I must be the gainer, "For to me," &c.

to live is Christ--whatever life, time, and strength, I have, is Christ's; Christ is the sole object for which I live (Ga 2:20).

to die is gain--not the act of dying, but as the Greek ("to have died") expresses, the state after death. Besides the glorification of Christ by my death, which is my primary object (Php 1:20), the change of state caused by death, so far from being a matter of shame (Php 1:20) or loss, as my enemies suppose, will be a positive "gain" to me.