Gaius was a dear friend of Apostle John and a member of a church (probably in Asia Minor)which John supervised. In his letter, John commends Gaius for his hospitality to traveling preachers of the gospel, for his faithfulness and for his love. In the above verses, we read of John wishing him well--Beloved, I wish above all things that thou mayest prosper and be in health, even as thy soul prospereth.
Sickness and weakness plague all who are born on earth and even the strongest are not spared. But a believer in Jesus Christ has a hope that one day his/her mortal earthly body will be changed to a glorious body that never ails or is weak. That is a thought worth cherishing. As the Apostle Paul writes to the believers in Corinth..
For we know that if our earthly house of this tabernacle were dissolved, we have a building of God, an house not made with hands, eternal in the heavens. (2 Corinthians 5:1)
John wished above all things that his friend Gaius may prosper and be in health as his soul was prospering. We can wish the same for our brothers and sisters in Christ. Though they be suffering, a few kind words can bring healing to their minds, if not their bodies.