Yes, you can definitely join DRDO after a B.Tech. in Computer Science.

• A Bachelor's Degree in Science or Technology (B.Tech or equivalent) in a relevant field like Computer Science is a basic requirement.
• First-class division with a NET (National Eligibility Test) or GATE (Graduate Aptitude Test in Engineering) qualification is typically preferred for Scientist 'B' posts which are entry-level positions for scientists in DRDO.

While GATE/NET is often preferred, DRDO conducts its own entrance exam for some  (padh - positions) or grades like Scientific Assistant and Technical Assistant, which may not require GATE/NET.

You can refer to this link for more information : https://engineering.careers360.com/articles/drdo-recruitment-through-gate