Were gathered together (sunēchthēsan). First aorist (effective) passive indicative. The church is not named here as in Act 15:4, but we know from Act 15:12-22 that the whole church came together this time also along with the apostles and elders.
Of this matter (peri tou logou toutou). Same idiom in Act 8:21; Act 19:38. They realized the importance of the issue.