Yes, the NIOS (National Institute of Open Schooling) high school marksheet is valid for applying to the Indian Post GDS (Gramin Dak Sevak) BPM (Branch Postmaster) position. NIOS is recognized by the Ministry of Education and its certificates are treated equivalent to those from other recognized boards like CBSE or state boards. Therefore, NIOS qualifications are valid for various government jobs, including the GDS posts under the Indian Postal Service.

However, make sure you meet all other eligibility criteria such as age, domicile, and knowledge of local language, which are also important for the GDS/BPM application process.