I have an opportunity to do internship at the cost of a gap in my academic year. Should I accept it? I have just completed my B.Sc in Physics and I have a unique opportunity to do internship in my field of interest for more than 6 months. If I choose it, I may not be ble to start with my Masters' courses this year and this academic gap will be reflected on my CV. 
I would like to know how will it affect me and my career. My aim is to become a good researcher.
You will have to weigh the pros/cons for yourself. There is no right answer to this and will vary for everyone.

Potential benefits:


More time to build your CV to apply to better universities
Real-world experience
Far higher income than graduate assistantships (at least in STEM in the US)
May provide you insights into your future research directions
Possibility of publications and patents if the organization does research
Networking that you may not normally be able to do (and letters of rec)


Potential downsides:


Delays grad school which probably delays graduation and getting a full-time position
If you have already been accepted to grad programs, they may not let you postpone them
Could potentially be a distraction from your long-term goals


Note, I did 5 internships during grad school so I obviously value internships. I do suggest to be strategical about internships to see if they are taking you closer to your goals. If so, go for it!