Hello Student
Yes you can do GNM course of 3 years and 6 months course although you are a arts of 12 class pass candidate. For taking admission in GNM course of 3 years and 6 months candidate has to pass 12 class with any stream arts commerce and Science with at least 40% marks in aggregate and percentage. There will be treated as General nursing cum midwifery courses for clinical nursing courses. Students from arts group treated as the same of  12th science  pass candidates for GNM courses. This Course is of 3.6years out of this 6months for internship.
Hope this help.