Gate will be easy as it will comprise the questions from your 4 years of under graduation.
As of DRDO, it is related to research and development  so the questions will be alot more difficult. In comparison GATE will be easy.
But if you get concepts and grab the skills to solve numericals then you can crack any exam. So go for your interest and grow your skills in it.
Hope it helps!

Graduate Aptitude Test in Engineering

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