CAT

At 50 percentile it will be difficult to get top tier colleges. You can look for one more attempt for CAT and other MBA entrance exam. At this score you may try for state engineering colleges MBA department. You can also look below link for this year colleges predictor-
https://www.shiksha.com/mba/resources/iim-call-predictor.

No you need to prepare for GK as well. There is sectional cut off for GK in IIFT. Yes, preparation for CAT and IIFT is similar. You can check the syllabus for IIFT on below link
https://www.google.co.in/amp/s/www.shiksha.com/mba/exams/amp/iift/syllabus
Also check this link-
https://www.google.co.in/amp/s/www.shiksha.com/mba/exams/amp/iift.

You need more than 95 percentile in CAT to get a call from IIM.

State entrance exams may help you. According to government rule for MBA atleast 1 competitive exam scorecard is required for admission.

You need to attempt a specific number of questions correctly. However, you need to overcome the situation where you are performing well in one section as compared to other due to the paucity of time. You need to look for questions taking less time to solve. This comes through regular practice. Join a good test series and evaluate your performance.

Your 12th percentage is very less and that will affect your entry to IIMs. You have to score between 99 to 99.5 for a safe bet and yes, graduation percentage matters.

You can either go for self-preparation if you find the topics difficult or not able to regularly practice the problems than better to join a coaching institute. Also, do join a good test series to evaluate your performance. Test series will give you an idea of time management, section selection and performance comparison with others.

Number system: Basic topic of this.Arithmetic: Arithmetic has a vast number of sub-topics.Algebra: Considering this has been one of the most dominant topics in CAT in the last three years, it makes a lot of sense to place extra emphasis on this topic.
Geometry: Another topic which has featured quite a lot in the last few years.
Permutation and Combination/Probability: This is another topic that contributes around one to two questions to the exam.
Miscellaneous Topics: This includes Set Theory, Trigonometry and Logarithms.

Career Launcher
IMS
These are the best.
Apart from these go for online apps and appear for mock tests. Doing all these I got 93.5% in CAT and was selected by Doon Business School, Dehradun.