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.