The revolver in that scene was almost certainly intended to depict a personally owned weapon, and certainly not anything issued by White Star Line. It was of a small caliber (or calibre if we want to go British-English) and was likely intended to portray a crewman's personally owned defense weapon, as opposed to the larger 'service' caliber .455 Webley revolvers issued to the Officers. I think in all the official testimony given at the inquiries, there were only a couple of mentions of pistols even made. I do not think there was any testimony or account that led to the scene in the film; just a bit of artistic license to up the danger and drama of people trying to survive. As Mr. Puttemans said, Lowe did say he carried what had to be another personally owned weapon. It's plausible there were more crewman and passengers who had their own.
I would guess it was also in there scene to show they were trying to beat back the passengers so they could lock the gate. There was a seaman there who was using an awe handle as a club at the same time the steward produced his revolver. As part of the plot device, they needed to establish that route was impossible to escape through. That was supposed to be the scene where we catch up with Fabrizio and Tommy, who had been absent for a few minutes.