I am a Pakistani national, however my girlfriend is from India we met in third country, now she is back in India, and I want to visit her, can I get the visa for India on basis of meeting girlfriend or should I have to make some other reason to apply.
Pakistani nationals can apply for a Visitor Visa to visit friends or relatives in India. This includes your girlfriend.
Because Pakistani nationals cannot obtain Tourist Visas, you should apply for a Visitor Visa and explicitly declare that you wish to visit your girlfriend. She will need to send you a sponsorship certificate that you will include with your visa application.
You can find the required documents and visa application process on the High Commission of India in Islamabad web site.