after finishing my undergraduate degree within how many years i can apply for gate exam??and how many attempts do i have?

Hello,
There is no age limit to appear for GATE and you can appear it for any number of times.The eligibility criteria is that you must have have passed Bachelor's degree or Master's degree in a relevant discipline from a recognised board or university.Candidates who are in final year are also eligible to appear.

So you must pass your Bachelor's degree and you can apply for it in any year after passing your graduation.