If you scored 258 on GATE Physics and got an interview call, that is good news. Even if the score is low, being shortlisted for IISc Bangalore shows that your profile is suitable for evaluation further.

IISc and other leading institutes consider additional measures besides the score from the GATE. How well someone performs, what subjects they studied, what purpose they share (if necessary) and, sometimes, prior research done may be decisive. Review the main topics of Physics and have an answer ready for any research or project you have worked on.

As a result, being shortlisted for GATE indicates that your background is seen as promising, despite not getting a very high score. Do your best in the interview since it will largely affect your final chances.