Entropy is an extensive property. If you double the size of your system but keep the conditions within it the same then you will double the entropy. You can't do this without changing other properties of the system because lots of things, e.g. free energy, are also extensive.
If you keep the size of your system constant but pack in twice as much gas then the entropy will change for two reasons, firstly because you have more gas, but secondly because you've changed the pressure and/or temperature. You need to specify the state of your system after addition of the gas to work out the change in the entropy.