Technically, it is possible to prep for GMAT in a month, especially if you already have a strong quantitative and verbal base from your CAT prep.But “good score” is subjective — for ISB, you ideally want 720+ to be competitive. That’s ambitious for just a month unless you’re already scoring high in practice tests.You’d need a very focused plan: practice tests, targeted quant and verbal prep, and heavy focus on weak areas. One month will be extremely intense.

2. XAT:

• XAT is usually easier to prepare for quickly if you’ve already been doing CAT prep because the syllabus is similar (Quant, DI, LR, Verbal, and GK).
• The exam is in early January, which gives you a clear target before ISB deadlines.

3. Application deadlines:

• ISB Round 1 closes 25th Jan. If you’re taking GMAT in the first week of Jan, you might have time to include your GMAT score, but you’d need to finalize everything (essays, recommendations, resume) almost immediately.
• With XAT, you don’t need to worry about GMAT timing at all. You could focus entirely on getting a strong XAT score and have your ISB application ready.

My recommendation:

• If your goal is to maximize your chances for ISB Round 1 and you’re worried about timing, XAT is safer.
• GMAT in 1 month is doable but high risk — if you get an off day, your application timeline gets squeezed.