Mr.sheraz, yes, I think the Invitation Letter from your Chinese girlfriend is necessary. Now I want to mention you that also need a Unmarried Certificate for marriage registration in China. You need first obtain a Unmarried Certificate from a local Notary Public Office in Pakistan, then the Unmarried Certificate need be verified by Pakistani Ministry of Foreign Affaires or other authorized organs in Pakistan, at last the Unmarried Certificate should be authenticated by Chinese embassy or consulate in Pakistan. Then the certificate can be used in China. Otherwise, you will suffer some difficulties when you two make marriage registration in China.