To specifiy initial conditions for your model, have a look at this answer and at the python scripts there included:
Temperature and pressure are scalar fields, so you will be able to repeat teh same procedure mentioned for both fields.
In general, your problem is complicated by the density dependence on both temperature and pressure.
To obtain your initial conditions, you should be able to apply the same conceptual procedure you would have used with the other software.
If you cannot, in the worst case, you compute the temperature and pressure field with the other software, then import them into your OGS model as initial conditions (using the approach posted above)…