Do gap years between 10th and 12th affect chances of getting placed after B.Tech?

No, it doesn't matter 

No, In the B.Tech placements, the only thing that matters is your skills and what you learn in your four-year college.
CGPA also plays a very significant role in placements.
In your first year, you might be thinking that you are behind by so many students but believe me it doesn't matter at all, the o

According to my view point in the placement interviews the first round questions will be in basic form which are been learned between 10th and 12th so there will be no affect in the gap while placements