B757 pilot reported the aircraft rolled uncontrolled while parking at the gate due to a misunderstanding with the ground crew.

Narrative

We pushed back from the gate much slower than normal. At the end of the push the person on the inter-phone said set brakes". I set the parking brake and said "parking brake set". For several minutes the ground crew tried to disconnect the tow bar. Eventually the person on the inter-phone told me to release the brakes so they could get the tension off the tow bar and that they would have to push us back a little farther. I said "brakes released". They pushed us back a little. Then they pulled us forward a little. Then they pushed us back a little more. Then we heard a loud noise. The aircraft started moving forward. I wasn't sure if we were rolling forward or if they were pulling us forward as the ground crew was not communicating with us. A few seconds later the person on the inter-phone said "break; break; break". I took that to mean "break away" which is what the flight operation manual says the ground crew is supposed to say in a situation like that. So I stopped the airplane with the brakes and then set the parking brake.The crew that pulled us into the gate told us to set the brake at the gate. Then they told us to release the brake so they could disconnect. I told them to standby so that I could make sure all passengers were seated. Once I was sure all the passengers were seated I released the brakes and told the ground crew the brakes were released. Then they just walked away and told us nothing. I had to call Maintenance and Ops to confirm that it was OK to turn off the seatbelt sign. The communication with the ground crew was extremely poor and non-standard. I shouldn't have to guess what the ground crew wants me to do in a situation like that. Also; the communication between myself and the ground crew remained poor and non standard all the way to the gate. This was a very dangerous situation caused by lack of communication and non standard communication by the ground crew that resulted in damage to ground equipment and the aircraft."