For to me to live is Christ
Christ was his life "efficiently", the efficient cause and author of his spiritual life; he spoke it into him, produced it in him, and disciplined him with it: and he was his life, objectively, the matter and object of his life, that on which he lived; yea, it was not so much he that lived, as Christ that lived in him; he lived by faith on Christ, and his spiritual life was maintained and supported by feeding on him as the bread of life: and he was his life, "finally", the end of his life; what he aimed at throughout the whole course of his life was the glory of Christ, the good of his church and people, the spread of his Gospel, the honour of his name, and the increase of his interest; and this last seems to be the true sense of the phrase here;

and to die is gain;
to himself, for death is gain to believers: it is not easy to say what a believer gains by dying; he is released thereby, and delivered from all the troubles and distresses of this life, arising from diseases of body, losses and disappointments in worldly things; from the oppressions and persecutions of wicked men; from indwelling sin, unbelief, doubts, and fears, and the temptations of Satan; he as soon as dies enters into the presence of God, where is fulness of joy, and is immediately with Christ, which is far better than being here, beholding his glory and enjoying communion with him; he is at once in the company of angels and glorified saints; is possessed of perfect holiness and knowledge; inherits a kingdom prepared from the foundation of the world, and wears a crown of life, righteousness, and glory; enters upon an inheritance incorruptible and undefiled; is received into everlasting habitations, into mansions of light, life, love, joy, peace, and comfort; is at perfect rest, and surrounded with endless pleasures. This is the common interpretation, and is countenanced by the Syriac, Arabic, and Ethiopic versions, which read, "to die", or "if I die, it is gain to me": but instead of reading the words as consisting of two propositions, they may he considered as one, and the sense be either this; Christ is gain to me living or dying in life or in death; for Christ is the believer's gain in life; he is all in all, his righteousness, his wisdom, his sanctification, his redemption, his life, his light, his food, his raiment, his riches, his joy, peace, and comfort; he is everything to him he wants, can wish for, or desire: and he is his gain in death; the hope he then has is founded on him, and the triumphs of his faith over death and the grave arise from redemption by him; his expectation is to be immediately with him; and the glory he will then enter into will lie in communion with him, in conformity to him, and in an everlasting vision of him: or thus, for me to live and to die is Christ's gain; his life being spent in his service, in living according to his will, in preaching his Gospel, serving his churches, and suffering for his sake, was for his glory; and his death being for his sake, in the faith of him, and the steady profession of it, would be what would glorify him, and so be his gain likewise; and this seems to be the genuine sense of the words, which contain a reason of the apostle's faith, why he was persuaded Christ would be magnified or glorified in his body, whether by life or by death.

Taken from John Gill's Exposition of the Bible

Philippians 1:21 In-Context

19 for I know that through your prayers and God’s provision of the Spirit of Jesus Christ what has happened to me will turn out for my deliverance.

20 I eagerly expect and hope that I will in no way be ashamed, but will have sufficient courage so that now as always Christ will be exalted in my body, whether by life or by death.

21 For to me, to live is Christ and to die is gain.

22 If I am to go on living in the body, this will mean fruitful labor for me. Yet what shall I choose? I do not know!

23 I am torn between the two: I desire to depart and be with Christ, which is better by far;