Hello.

GSEB is a State Board whreas CBSE is a Central Board. GSEB Board syllabus is confined only to that particular state whereas CBSE Board syllabus is same throughout India. So, it is best to study from CBSE as you can face any competitive entrance examinations. 

But overall as GUJCET is a State level exam so the syllabus will be exactly from GSEB board and it doest mean that CBSE is waste it is good but little more syllabus than GSEB.

However, staying in the same board will prove to be beneficial for you as the examination and education pattern will be the same. Moreover, in most of the entrance examination nowadays CBSE examination pattern and syllabus is being practiced. Therefore, it is one such reason to remain in the present board.

The conclusion for your question is studying 12th completely is enough to crack GUJCET.

Hope this is helpful.

Good luck.