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Hello,

I am not clear as to whe you have done you diploma, as minimum qualification for dimploma course is passed your 10th exam.

if you have done the course after 12th then you are eligibile because there is no age limit in GCET 2019 for the candidates. 

Minimum eligibility for an engeenering course is: Candidates must have pass 10 + 2 or equivalent examination from recognized board. He/she should have Physics, Mathematics and English as mandatory subject and any one of following additional subject in Chemistry/ Biotechnology/Biology/ Computer Science. Candidates must have scored at least 45% marks aggregate for general group and 40% for SC/ST/PH candidates.

hence, look into it accordingly