Marriage

You are Christian, and you cant marry this person, such fall into sapinda relationship.  Legally it is wrong and also medically it will lead to complications like major birth defect in new born. Marry someone outside of the famliy tree.  Marrying anyone inside of family tree is not allowed according to Indian law.

but the boy is from maternal side, and gotras are also different. This case is from South India. they are in true love, is there any solution to be together? please help

I do agree marriying within family will lead to medical complications. If they are very particular, let them consult a Doctor and undergo medical tests. As per doctor advice they can think about their marriage.