What is the reaping that God is doing in this passage?
The reaping is God's sovereign judgment: Christ thrusts in His sickle and the earth is reaped. Then another angel gathers the clusters of the vine of the earth, whose grapes are fully ripe—overripe—to be crushed in the vat. This reaping involves the nations being called to the Middle East.
Les explains that the communication is between the angel and Christ. The angel tells Christ to thrust in His sickle and reap because the time has come, and Christ does so, reaping the earth. Les connects this to chapter 16, where the call goes to all the nations of the world to bring their armies and air forces to the Middle East; that, he says, is the reaping the sovereign God is bringing about. Then another angel comes from the altar, who had power over fire, and cries to the one with the sharp sickle to gather the clusters of the vine of the earth, for her grapes are fully ripe. Les notes that 'fully' in the Greek means overripe, past harvest time. The grapes are gathered and put into the vat to be crushed, the juice taken out, and then fermented for wine.