The petrol gage is not working how do I fix that it is a Chevrolet captiva ltz 4x4 2008 model

Unfortunately, it's usually the sending unit that's gone bad, it's inside the gas tank.
Very few vehicles have an opening in the floor directly above the gas tank to pull the sending unit and pump. Usually you have to jack up the vehicle and drop the gas (petrol) tank.
The replacement part is cheap and a mechanic should be able to do the repair for a decent price. They have to remove most of the petrol before removing the tank so I would get the tank close to empty before bringing it in. (I know, without a gage it's hard to guess)