Quick question about Gibbs free energy

[Zarquon]

In the expression defining Gibbs free energy,
G = U - TS +pV,
are T and p the temperature and pressure of the environment, or of the system itself? Or is it a requirement that the system has the same temperature and pressure as the environment for the Gibbs free energy to be defined?

 

[tannerbk]

T and p describe the system. For a system in thermal equilibirum and mechanical equilibrium with the environment - consider the system surrounded by heat and pressure reservoirs - the Gibbs free energy is at its minimum value.

 

[Zarquon]

Thanks.
Part of the reason for my confusion is this page:
http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/helmholtz.html
Seems to me that the "work to give the system final volume V at constant pressure p" should be equal to pV where p is the pressure from the surroundings?