Cracking GATE is not a tough task. You can always score the cutoff

What is tough is securing a rank in top 1000 so that you can grab a seat in one of the NITs or IITs/deemed university, otherwis it isn't worth going for an M.Tech. And for that, you need to have clear basics, which is not possible in one month. I don't want you to lose hopes but that's the truth. Even if you start about 3 or 4 months earlier you have good chances of scoring in top 2000-5000, depending on your capabilities and hard work. Anyhow, even if you want to start now, you need to make a plan

Step 1: Go through atleast last 10 year question papers and you'll get a taste of what you going to deal with and the extent of the syllabus plus the important areas you need to stress on. Step 2: Make a priority subjectwise list of topics which carry most of the weightage of the paper in a decsending order. Step 3: Classify the topics into the ones that you are good at, the ones you need to work on a bit, the ones that you have no idea about. Step 4: Optimize and create a final list of topics which can be covered in the short period of 20 days. Step 5: You must save atleast 2 weeks(here you've only got 4 weeks so better make it a week/10 days) for revision and mock tests.

This is to judge your preparedness before taking the exam, otherwise you may feel hopeless on the exam day when you see the flood of questions. Its easy to solve questions subject wise or topic wise because you've just read the topic and you know which formulas to use, not so easy when everything is mixed up in a single paper.