Peter Singerís moral system can be generally be defined as a Utilitarian approach to ethics. That is, that morality is defined as that which brings pleasure and reduces pain for as many beings as possible. Where Singerís theory departs from Bentham and Millís classical Utilitarianism is where Singer goes beyond pleasure and pain and looks at the interests of sentient beings. Thus, Singer bases his position on several ethical issues on whether or not they suit the interests of the majority of living beings.
Gandhiís moral system was born in his ďexperiments with truthĒ. That is, the trials and tribulations throughout his life and what he learned from them by being steadfast in truth and integrity. Gandhiís moral system was also influenced by different religions including different forms of Hinduism and Christianity.
As far as differences between the two, the conflict begins in that Singerís approach is more of a pragmatic one. Singerís philosophy is based more on the interests of people and less on abstract ideas of morality, and seems to be free of any religious ideas. For example, Singer argues for legal abortion on the grounds that it suits the interest of the mother who does not want the child, not based on the moral implications. For Singer, what is moral is practical and what is practical is moral. If Singer could have been placed in the shoes of a young Gandhi, and faced with the same dilemmas, he would have been more likely to indulge in wine and woman (I say woman, based on Singerís ideas about casual sex) since there was no real impending consequence. Singer might not have found the purpose in unnecessarily depriving oneself of these things. I would imagine that Gandhi would be appalled by the practice of abortion, of which Singer approves. Gandhi never failed to take responsibility for his mistakes (if they can be even considered his mistakes in some occasions) and he would probably see abortion as the failure to take responsibility.
Where Singer and Gandhi seem to be in agreement is on the issues of animal rights and poverty. Singer and Gandhi both argue that animals are capable of suffering just as we are, and that the life of an animal is no less precious than that of a human.† Gandhi did not see the justice in the use of animals for food and religious sacrifice, nor does Singer. And on the issue of poverty, Singer felt that those who are more fortunate should help those less fortunate for everyoneís benefit. Gandhi felt the same way when he saw that Indians who were making money in South Africa were sending it back to India where people were less fortunate.