For BSc Hons Chemistry the basic subject combination required by most central and state universities is Physics Chemistry and Mathematics in Class 12 along with English as a compulsory subject. Regarding GAT it depends on the admission process of the university because if admission is through CUET UG then students usually need to appear for Chemistry and Mathematics and sometimes Physics along with English while the General Test GAT is not compulsory for most universities unless it is clearly mentioned in the eligibility criteria. About NIOS dual marksheet yes NIOS marksheets are generally accepted including cases where students have completed subjects in two phases or with dual marksheets provided the subjects are passed from the NIOS board the marksheets are valid and recognized all required subjects like Physics Chemistry Mathematics and English are clearly mentioned and the university allows combining marksheets. In some cases universities may ask for an equivalence certificate or clarification at the time of admission so it is always advisable to check the official admission notification of the university before applying.