From which subject one should start preparing for GATE and for how much time a student should study on daily basis to get a good rank < 100 ?

GATE preparation usually requires a minimum of 7–8 months of dedicated preparation. If you start from May then it will be sufficient enough to crack it. If you start early then you get more time for revision and solving the variety of questions.

Total month = 7

Total Days = 210

If you read 10 hours a day then you can achieve a rank below 1000
It requires around 800 to 900 hours to complete 10 subjects.( Theory, Concept and 200 MCQ s of each)

So on average 70 -80 hour is necessary for each subject.

Since I am from ECE there are 10 subjects almost.

In every revision, you have to concentrate more on hard topics which are hard to remember and those which are frequently asked in the exam.

Easy questions can be solved by every student but hard questions can be solved by only a few peoples.

Manage your 24 hours properly.