Gibbs free energy relates to systems at a constant pressure.
Helmotz free energy relates to systems at a constant volume.
Note that volume can change when systems at constant pressure change; this leads to work being done on or by the system. Pressure times volume has the units of energy (or work). The model is a piston moving through a cylinder; it can compress air or be pushed by steam or burning gasoline.
Systems at constant volume can do no outside work.
Thanks Doc for clearing that up!