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 ?

First of all have a look at the syllabus.
Then be comfortable and make your mind focused. Because without peace less mind you cannot concentrate.
Then start studying the subject you like the most and you are confident in it try its all questions related to that subject. Do not indulge in it for your whole preparation time.
Then move to other subjects. You can shuffle subjects as per your mood and relaxation. 
All matters is you must have the concepts clear so that you are not stuck while attempting paper.
​All the best.!!!!!!