What was done with the gathered grapes in the wine-making imagery of this passage?
The gathered grapes were put into the vat where they could be crushed, the juice taken out of them, and then fermented or whatever they did for their wine. The passage notes they were fully ripe, even overripe, before being gathered.
Les explains that in verse 18 another angel came out from the altar, who had power over fire, and cried to the one with the sharp sickle to thrust in his sickle and gather the clusters of the vine of the earth, for her grapes are fully ripe. The word 'fully' in the Greek means overripe, really past harvest time. The symbolism is that the grapes were gathered and put into the vat where they could be crushed, the juice taken out of them, and then fermented or whatever they did for their wine. This follows the earlier reaping where the earth was reaped, and the call goes to all the nations of the world to bring their armies and air forces to the Middle East.