As I said, character called Newsreader (played by Ian McNeice) used those strange gestures, that looked like some form of sign language. Was this a thing in Republic/Empire? If yes, why they were doing that? If not, was there any set of gestures to communicate with deaf citizens of Ancient Rome?
You will find a great answer about this here (Credit to u/xenephontheathenian)
You can also listen to an interview with Ian McNiece talking about it here. He mentions that one of the historical researchers devised a pamphlet of what the gestures looked like which most likely came from the Institutio Oratoria which was a guide for orators. By the sounds of it, he based a lot of those on the original source while adding some of his own where appropriate.