If your question is if you can give GUJCET though you are reappearing for 10+2 exams,  even if you appeared for GUJCET in 2018, yes you should appear for GUJCET in 2019. The GUJCET score is only valid for 1 year and for the year that you wrote the entrance exam in. You cannot carry forward the score. You need to reappear for GUJCET.