Nursing course is different from a regular medical degree course and therefore biology subject is not mandatory for pursuing GNM course. The eligibility criteria set by the Indian Nursing Council for GNM course doesn't specify studying biology as a subject at 10+2 level. Even students who have passed 10+2 in arts stream can pursue GNM course.
Any male or female student having passed 10 + 2 passed with minimum 40% marks is eligible to apply.The candidate should have completed 17 years of age on or before 31st day of December of the year of admission. The duration of the course is three years.
Regarding admission procedure, it is better for the author to first decide as to in which state/city , the course is to be pursued. Private college also sometimes conduct their own entrance examinations, but generally students who are able to bear the cost, are able to get admission.
It is better for the author to study the website of Indian Nursing Council (http://www.indiannursingcouncil.org/) to know about list of recognized nursing institutions and various other related aspects of the education in nursing field.