A "ghetto" is a part of a city dominated by one or more ethnic minorities. "Gospel" is originally a Biblical text describing the life of Jesus. It is also used more widely to describe a text with a moral message, in this case the message is about life in the "ghetto".

People have already defined the terms. I think Tupac wanted people in the ghetto to know it was time for change. They needed to hear him and follow his lead out of the slums. Change begins from within, and maybe, this song was delivered in order to spread the word for peace. He questions the conditions of the streets in many of his songs. I think in the song he wants people to follow his lead out of the hood. People in the ghetto are people, just because they don't live 'pure,' it doesn't make them less in 'god's,' eyes. He was preaching, in a way he thought people would understand. I believe he hoped to inspire youth to not fall victim to the cycle of gangs or poverty - become new slaves, as he put it. "But am I less holy Cause I chose to puff a blunt and drink a beer with my homies Before we find world peace We gotta find peace and end the war in the streets My ghetto gospel."

Ghetto = slum, bad neighborhood, hood. This word is of African American origin.

Your dictionary should give you the meaning of "ghetto" and "gospel". 2pac is making a plea for peace, love and reconciliation - if you like a "gospel", to people in the ghetto and also to those outsiders who fear and despise the residents of the ghetto. At least that's my interpretation...