hi,

see Irrespective of your Gate papers, you are eligible to apply for any of the streams  or papers available in gate. it is not mandatory that you need to appear in Gate exam from the subject related with your bachelors course. you may do self study and appear in any of the papers. however gate score is evaluated to admission in masters course and psu hiring. so to admission in any of the course you must satisfy the college eligibility criteria and their required subject of Gate papers. so first decide after completing b.sc which of the course you are interested for and select the paper accordingly.

follow our article at below link to know in details:

https://engineering.careers360.com/articles/gate-eligibility-criteria