1) you have appeared as private student for SSC exams as your school was not recognised by SSC board
2) as per GAIL advertisment Only full time Regular courses will be considered. This shall include Class X & XII examination, all Diploma(s), Graduation & Post Graduation etc.
3) Before applying for the post, candidates should ensure that he/ she fulfils the
eligibility criteria and other conditions mentioned in this advertisement. GAIL would
be free to reject any application at any stage of the recruitment process, if the
candidate is found ineligible for the post for which he/ she has applied.
4) since you dont meet the eligiblity criteria your application would be rejected