GMAT

Some colleges can accept your admission without GMAT score. But, they will still take IELTS or TOEFL score. You will have to look for such colleges individually.

Being a 2nd year student you do have a lot of time to prepare as next year would largely be focused on your training and the final year on placement. Start preparing bit by bit on your own. Build a strong vocab in next few months followed by writing skills and than appear for a mock or two every month. This would ensure your continuous evaluation and would help to improve until the final day. Give your GMAT in the final year staring as this would help you with enough time to apply for the scholarships and other financial aid.

It is necessary to take exam, and yes good percentile is required to get the admission, as it is the top institution for management studies.

Yes, IIM institutes do accepts GMAT scores. But the validity for GMAT score is only 1 year.

It depends on college criteria and admission rule. Different colleges has different admission cycle and process for taking admission and GMAT session and duration.

According to me, you must go with CAT, as through CAT you will have much more better options and your 6 years of experience will be very helpful to you there.

Yes, for IIMB, you need a valid GMAT/GRE score.

CAT is mandatory for MBA admissions in Indian colleges. GMAT is exam required to gain admission to MBA courses in foreign universities.

To pursue MIM or MS programme, GRE is mandatory. GMAT is usually required to get admission in MBA programme. For USA, Canada and Germany, GRE is required. However, IELTS is required in Australia. Only some universities would ask for GRE for these programs.

The colleges which accepts GMAT for indian students are;
(1) IIMs (Particularly Ahemdabad and Banglore) for 1 year PGP program
(2) ISB Hyderabad.
(3) IIFT.These are the top notch institutions which will accepts your GMAT Score.
Hope this helps.
Good Luck.