hi friend,
firstly you have to understand that gas behavior deviates from oil behavior.
Gas is highly compressible so there is 2 problems
1.) Viscosity is a function of pressure
2.) Density is function of pressure

the concept of pseudo pressure is not new, in other disciplines of engineering this problem has been solved by substituting a function in to the original differential equation.

In the case of oil reservoir we dont have such problems , we have the diffusivity equation and the solution is well document viz. Ei solution.
The Ei solution directly gives pressure.

Now by using pseudo pressure function in case of gas , the diffusivity equation is in the same form , only difference is that we have 'Si' in stead of pressure.

We can use the solution, get 'Si' and reverse calculate P.
Regarding obtaining the 'Si' you must know the PVT properties and generate a table for a range of values of Si vs P.

P is the average reservoir pressure or initial reservoir pressure.

if still did not understand the concept i suggest you read the document " Gas well testing manual by ERCB".
I is available on scribd.com

Good luck