How to write a custom gas file to use in Geant-4 and garfield++ interface

Hello

I am modifying the Geant-4 and Garfield ++ interface example code to try and understand the workings of this interface. Now the .gas file used in this interface is ar_70_co2_30_1000mbar.gas, I want to use p10 gas which is filled with the pressure of 1 atm inside a cuboidal Proportional counter of dimension 600cmX10cmX10cm. This proportional counter has a central anode wire of length 600 cm and radius 50 micrometres on which 3000 V is applied.
Now how should I create a suitable gas file? (I want to study the effects of electric field on energy deposition)

Thanks

https://garfieldpp.web.cern.ch/garfieldpp/examples/gasfiles/