hi,

see the general eligibility criteria for GATE exam is candidates who are currently in final year of three years b.sc course/ bachelors degree  or third or 4th year course of b.tech or already completed graduation or masters or in second years of masters course are eligible to apply.

gate exam has no particular provision regarding selection of papers. irrespective of your bachelors course you may choose any of the gate papers. however you must first decide which higher degree course you intend to apply basis on your gate score, the eligibility criteria, whether it is applicable to you or not.

hope you understand.