I am in 12 th STD. I focus on Gujarat board more than all that entrance exam..then what if I score good in board and not in JEE..

NCERT textbooks are best suited for preparation of JEE Mains. Many chapters and topics of Physics, Chemistry and Mathematics of the state board would be similar to that of CBSE. If you can get yourself a set of NCERT texbooks of these 3 subjects (class 11 and class 12), it will give you a fair idea of which topics, if any, are missing in your GSEB and try to learn them too, so that you are simultaneously preparing for JEE mains too. Make a note of the JEE Mains syllabus and cross check in both your GSEB as well as NCERT Textbooks and prepare accordingly.
Note: JEE Advanced needs a different level of preparation

Also do note that many of the entrances like SRMJEE, VITEEE, Manipal are relativey easier and are mostly based on class 11 and 12 syllabus. Also GUJCET is based on your GSEB Class 11 and 12 syllabus only.
Make a list of all entrances you wish to appear, note down their syllabus and compare them with your GSEB syllabus and prepare those chapters accordingly.

In this condition, you can get through management quota into good government and state instituitions including private colleges as well. You can also apply in different entrance exams if you know you cant qualify JEE. Other competitive exams are not that tough to crack.

Regards,

Keshav Sahani