The word, “Free” in Gibbs Free Energy can be often confusing and I will try to address this aspect in simplified manner. Enthalpy is the total heat content, which means the internal energy + the product of pressure and volume. However, all enthalpy is not available to do work. The reason is the entropy or disorder. Nature seems to prefer disorder in broader sense as it does not like to spend energy to arrange things (there may be symmetries and orderliness in local/system/surrounding context but in broader context, i.e. in the context of the universe/multiverse, the disorder wins). Some enthalpy is spent in entropy. The remaining is left to do work and because that is what is available to do work, it is called available energy or free energy.