To die is gain for Paul, but to continue living is a different kind of gain. Here he calls life living in the flesh
which is life on earth. If he lives, he can do fruitful work. He can continue to preach the gospel. That will bear fruit: more people will believe in Christ. If he had to choose between living and dying, he would not know what to choose.
22 If I am to live in the flesh, that means fruitful labor for me. Yet which I shall choose I cannot tell.