Hi welcome to the Community, you and your girl friend can try getting married, she can come to Nepal to marry you there that would be the easiest option of getting Married or you can decide to apply for German Marriage Visa and then marry in Germany. If you choose to Marry in Nepal, after successfully marrying there you can apply for family reunion Visa to join your wife. and then after arriving in Germany you can then apply for the German Residence Permit. The Residence Permits are normally issued for a duration of 3 years after then you can apply for Permanent Residence Permits or German Citizenship. If you are still legally married and if you meet up with the requirements like having adequate knowledge of German e.g B1level, Tax and other requirements. Or if you got a Marriage visa to Marry in Germany after successfully marrying in Germany you can also then apply for the German Residence Permits same rules apply towards your Residency status
Best regards
Nelly