The owner of the car was inside the store and the witness said he believed Gull had locked the doors.
The witness said he tried to keep his eyes down and zipped up his jacket to hide his shirt, which happened to be red and white.
"I was taking every precaution."
He said he saw at least one object that could have been a hatchet or a hammer swung into the back windscreen, and saw the others "attacking the side of the car".
"I was worried someone was in the car."
The back and side windows were smashed.
After a long couple of minutes, the men got in their own car and took off.
The witness did not see what kind of car they were driving as he shuffled to the damaged vehicle to try see if someone was in it.
He tried to see which direction the fleeing car headed when someone ran to the damaged vehicle and "screamed out of there".
A police spokeswoman said police were called to five or six people fighting on the petrol station forecourt.
During the fight glass was smashed, and the people involved left in two vehicles, she said.
No injuries were reported.
Police spoke with a staff member at the Gull and are making follow-up inquiries, including whether there may be CCTV footage.
There have been no arrests.
A Gull after-hours call centre spokeswoman said nobody would be available for comment until Monday.