It is just the voltage from a battery (electrochemical cell)!
Soon I will give a lecture introducing the free energy in a thermodynamics course. It is an incredibly important concept. We try and get the students to learn the course mantra, "the Gibbs free energy must be minimised (at constant temperature and pressure)".
Two significant points for students to learn about the Gibbs free energy
-it is directly measurable
-where many of the tabulated values for delta G for chemical reactions come from
The voltage of an electrochemical cell V (in Volts) is related to delta G (in Joules per mole) by
V = -delta G/n F
where F = Faraday constant = 96485 J/mol/V and n=number of electrons transferred at the electrode.
In my lecture I will show two really nice videos from Chemistry Comes Alive on the electrolysis of water. I particularly like how the second video (below) shows how the ratio of the volume of the gas produced at the cathode (hydrogen) is twice that produced at the anode (oxygen).