|"When i play games for 1-2hours the GPU temp can reach 70-72 Celsius"|
For a GPU under load, that's really not that bad. Most GPUs can handle up 100C+. I think you're worried about a problem that doesn't exist. Just make sure the cables inside the case are out of the way so that they don't obstruct airflow. For cooling fans, all you really need is one exhaust fan in the back. The power supply (hopefully it's mounted on the top) also helps to exhaust air from the case. Side panels fans are disruptive & should not be used. Front intake fans are optional. Test have shown they are of little benefit. Check this article, it's a bit old, but it still holds true.
As for laying the case on it's side - it won't hurt to run the system that way, but there's no guarantee it will run any cooler. The tower design is used because warm air rises, then the exhaust fans near the top of the case blow the hot air out the back & at the same time, create a negative pressure within the case which causes fresh air to be sucked in thru the front. BTW, leaving the side panel off generally renders the case fans useless. They need a closed case to do their job.