In considering social justice and the social gospel, I think that we need to look to Jesus' example. He came to usher in the Kingdom of God, and the beginnings of that Kingdom could be seen in His work. His ultimate goal was to die and rise again for our salvation, but He spent several years preparing for and working towards that goal.
What did Jesus do in His ministry? He went about preaching and teaching about the Kingdom, healing the sick, casting out demons, healing the blind, deaf, lame, and mute, and on occasion feeding the hungry and raising the dead. Most specifically, look at the majority of His healings when the Gospels go into detail about what happened. Jesus does not fail to deal with the spiritual needs of the people. He pronounces forgiveness, He encourages faith, but He also actually deals with their spiritual needs. If nothing else, the setting right what was wrong about their situation was a sign that the Kingdom of God was at hand.
In our proclamation of the Gospel, we are proclaiming that the Kingdom of God is at hand. That Kingdom is seen most preeminently in the death and resurrection of Jesus and the calling of people to faith in Jesus. Our "doing" of the Gospel dare never neglect that. But as Jesus cared also about the physical wellbeing of the people with whom He interacted, so part of our "doing" of the Gospel needs to be care for the physical wellbeing of the people to whom we preach. That care should also be a sign of the Kingdom breaking into our world. To neglect caring also for the physical needs of people as well as their spiritual truncates the Gospel message.
I would note, however, that caring for the physical needs of people does not necessarily mean that we must do so according to the doctrine and teachings of Marx, Adam Smith, Democrats, or Republicans. Nor is it an adequate proclamation of the Gospel when human, temporal needs are met without also giving spiritual aid. Jesus did not say that simply giving a cup of cold water was praiseworthy and deserving of reward, but giving a cup of cold water in His name.