See admissions are not on GSEB board marks for class 12. You have to appear for GUJCET which is the entrance exam. The eligibility to write GUJCET is " The aspiring candidates should have passed Std. 12 (Science Stream) with minimum 45% (40% for SC/ST and SEBC) in three subjects viz two compulsory subjects i.e. Physics and Mathematics and any one of optional subjects i.e.  Chemistry/Biotechnology/Biology/Technical Vocational Subjects prescribed by respective boards) from single board and should have appeared in GUJCET 2017 to become eligible for the admission."

I am quoting this directly from the official notification. As for GUJCET, check all details here please - https://engineering.careers360.com/articles/gujcet