Under the following Indian laws, you cannot marry before the age of 21:
Special Marriage Act: Sec 4(c) - the male should have completed the age of 21 years and the female the age of 18 years;
Hindu Marriage Act: Sec 5(iii) - the bridegroom has completed the age of 21 years and the bride, the age of 18 years at the time of the marriage;
The Prohibition of Child Marriage Act 2006: Sec 2(a) “child” means a person who, if a male, has not completed twenty-one years of age, and if a female, has not completed eighteen years of age;
The Foreign Marriage Act, 1969: Sec 4(c) the bridegroom has completed the age of 21 years and the bride the age of 18 years at the time of the marriage.
Thus, you WILL have to wait until you turn 21, to legally marry your girlfriend.