How to properly pressurize gas pipes?

I think the over pressurisation is on the room side of the vent. Try venting bad air out of the room.

You can only jam so much air into a room without off venting.

Gas vent being overpressurized means the room you're trying to pump the air in (the tile the vent is at to be exact) already has too much gas in it. Currently however the game has a bug where the warning does not go away even when the vent is working fine. For the vent to really stop working due to being overpressurized (you will see the air starting to build up in the pipes and not going out) you need well above 2kg of gas in the room per tile.