The verse doesn’t mean these differences don’t exist. Clearly males, females, Jews, Greeks, slaves, and free people exist and are different from each other in some way. What the verse is saying is that these differences don’t define believers compared to their unity in Christ. 
And most critically for this conversation, with the following verse, “And if you belong to Christ, then you are Abraham’s descendants, heirs according to promise” it’s hitting home that regardless of physical bloodline, all Christians are Abraham’s descendants in spirit and covenant. 

 I know the verse doesn’t mean those differences don’t exist. The term Christian means follower of Christ. Both Jews and Gentiles/Greeks were followers of Christ. Jesus himself was not a Christian because he is literally the Christ. 

 Jews do not believe that Christians are descendants of Abraham and do not like Christians. Jesus litteraly said that if you reject the son, you hate the father. Jews reject the son very harshly and therefore hate God