Hello

Yes, you can definitely leave your gnm course if you are are getting bsc nursing and you want to pursue it.

But yes,

You need to check  with your college once.

This is because,

There's possibility of having a bond in your college according to which you might have to pay a certain amount of money as penalty for leaving the gnm course in between and wasting a seat.

Other than this, there's no problem at all.

Regards

ADITYA KUMAR