How does Le Chatelier's principle work in a gaseous system?
Le Chatalier's principle states that if a stress is applied to a system at equilibrium, then  the system, will shift its equilibrium position in order to bring the system back to its equilibrium.

So the stress applied to the system could be temperature, pressure and the concentration of one of the components in the system.

I will choose to explain about pressure because apparently you have the problem understanding that.  

Pressure.
If you add pressure to a system then the stress will be to decrease the pressure by shifting the equilibrium position towards the side with less number of particles.
If you  remove pressure from a system, then the stress will be to increase the pressure of the system by shifting the equilibrium position towards the side with the higher number of particles. 

So it is true that when you increase the increase the pressure in a system, then the system is stressed to reduce the stress. In order to do that, the system's equilibrium will shift to the side with less number of particles.

For example:

2Fe$_2$O$_3$ + 3C -----> 4Fe + 3CO$_2$ 

So if you add pressure to this above system, then the stress is to reduce the pressure. On your reactants side the sum of the coefficients is 5 and on your products side, the sum is 7. Since you need to bring it down then the shift will be to the side with the least sum which is to the left-hand side. Therefore, the shift will be to the left-hand side.

Hope this helped you with the problem.