See you should apply for jee mains as it will give you great opportunities to find you a great college as per your interest in space science, astronomy through engineering fields. Scoring good in JEE will help you open vast opportunities in future.

GUJCET is basically state based exam if you dont want to move out of Gujarat then it would be beneficial to you to get seats easily in home state and if you have planned to move out of Gujarat for studies then  its upto you to give GUJCET or not.

Both the exams will help you achieve your dreams and you should do what you are interested in.