Congratulations on securing an interview with an IIM! In terms of extra courses or certifications, it depends on the specific program, your interests, and your passion. Some of them are listed below
Basically any course which develops your leadership, communication, marketing, finance skills gives you an edge over other appearing for IIM Calcutta admissions. However, remember that if your resume shows a lot of courses, then you will have to face the consequences in the interview. The panelists of IIM Calcutta are well aware of the fact that candidates do these types of courses just to make their resumes appear better.
The selection criteria for IIM Calcutta differ greatly from those of the other IIMs. Calcutta gives CAT score and X, XII marks weightage for WAT and PI selection.
Because Calcutta places a high value on CAT scores, it is advisable to begin preparing for the exam as soon as possible. Give a lot of practice tests, evaluate your results, and figure out where you're going wrong.
Once you have a good CAT score, you should start preparing for WAT/PI. Because your GPA is low, there is a good chance that the interviewer will be concerned. Make the interviewer understand why your GPA is low, and everything will be fine. You will get in if you work hard enough.