‘But the Pharisees, when they heard that he had put the Sadducees to silence, gathered themselves together.'
The Pharisees had no doubt heard with approval that Jesus had confuted the Sadducees on their favourite topic, but it only stirred them up the more to try to show Him up. So they came together again for that purpose (compare for ‘testing' Jesus Matthew 16:1; Matthew 19:3).
‘Gathered themselves together.' Compare Acts 4:26 citing Psalms 2:2. The idea is of gathering together in patent hostility.