Hello,
Yes you can go for MBBS after your GNM, i.e, General Nursing and Midwifery course. GNM is a three and half years diploma course.
If you have had physics chemistry and biology as subjects in class 12 , then you can apply for NEET exam and then try for admissions into MBBS as long as you satisfy all the eligibility criteria for admission.
Candidates must have completed17 years as on December 31, 2020. There is no upper age limit till the Supreme Court passes its judgement on it. There is no limit on the number of attempts a student can make.
The qualifying marks in class 12 are as follows
PCB subjects
UR - 50%,
OBC/SC/ST - 40%,
PWD - 45%
Please visit this link for detailed eligibility criteria for NEET exam

Hope it helps