EPro is represented by 'an eye for an eye and a tooth for a tooth'. If someone commits a criminal wrong against someone else, the punishment should be equal against the offender, sometimes taking the same form. It is intuitive and has the strength of an appeal to justice, balancing the scales.
LPro maintains that the punishment should be proportionate, but it denies that it needs to be the equal. This is sometimes justified by saying that a state's punishment is justified because it censures and condemns the offender, and it sends a message to the offender and others that the act is not tolerated. LPro theorists also sometimes argue that strictly equal punishments are barbaric. How is a government justified in doing to the offender what the offender did to others if it is an offensive act?
GPro is less popular than I think it should be. I do not know of a pure retributivist who advocates GPro. How would one justify the GPro position from a purely retributivist perspective? I think the general flow of the argument is pretty simple. A infringes on one of B's human rights without provocation, and this offence has a severity of 2, on a scale of 1-5. This is wrong; it is unacceptable. The state with jurisdiction should, after a fair trial resulting in a jury verdict and so forth, find a sentence with a severity of 2. However, it should sentence A to more than a level 2 punishment. This is due to its justified role in punishing such an offender. An equal-consequence response by the state to an offence would not be equal because the offender was not justified in his infringement on B's human rights but the state is justified in its punishment. Therefore, in order for it to be equal in all respects, the punishment must add to the equivalent-consequence sentence an addition for the unjustified violation of B's human rights. So, GPro can be justified if punishment is understood to be properly equal to unjustified crime.
