so what gain is he referring to?
We discover, as we grow in Jesus who has it all. We share with Jesus and one another. Prosperity, then, includes sharing, not only possessing materials, as you have said.
And it is not about controlling and using people >
"nor as being lords over those entrusted to you, but being examples to the flock." (1 Peter 5:3)
"Assuredly, I say to you, there is no one who has left house or brothers or sisters or father or mother or wife or children or lands, for My sake and the gospel's, who shall not receive a hundredfold now in this time---houses and brothers and sisters and mothers and children and lands, with persecutions---and in the age to come, eternal life." (in Mark 10:29-30)
"a hundredfold in this time" > including of people > mothers, sisters, children, brothers < family people.
With Jesus, if we leave all for Him and the gospel, we will receive "a hundredfold now in this time" of family people . . . for family loving and sharing, I can see. Our Christian prosperity, includes family sharing and caring with all our Jesus family people.
But this comes "with persecutions". We need to be strong in God's love so we can enjoy being with one another, able to forgive "even as God in Christ forgave you" (in Ephesians 4:32), and so we can stay this way during "persecutions". When God blesses us, Satan may be allowed to test us with the exact opposite, and then are we with God, ready for loving still?
Ones of our dearly cherished Jesus family will die before we do, and they will have different problems of what Satan is allowed to do with their unbelieving relatives and friends and to them, themselves. So, in order to do well with God's prosperity shared with us we need to grow in Jesus and His love's creativity so we can take good, godly advantage of all which will happen . . . like how Joseph did > Genesis 37-50 < helping save the lives of many people, but also reconciling with those brothers who sold him into slavery in Egypt.
Are we ready for love, with any and all people . . . like Jesus on the cross loving any and all, having hope for any person, at all? This comes with how our Father corrects and transforms our character to be godly in His love > so we have godliness of character.
This makes us intimate with God Himself and all-loving; this is great gain of godliness, I would say.