For an isobaric process, we wish to use the 
Enthalpy, for the pressure appears in the 
differential there.  This choice allows us to carefully account for the 
work done on the system and that done on the environment in a process.
For a process that is both isobaric and isothermal, it makes sense to 
look at the Gibbs Free Energy.
Therefore, while solving problems, look for what is being held constant 
so that you can make an appropriate choice of energy.