There is nothing like that. GATE exam in two shifts per day as per the schedule. Some subjects have less students and their exam happen in one shift at a particular day. Some have more number of students and their exam can strecth to two shifts a day or to two days max.It is not like the exam of one subject will be held on all 4 days. It will be held on a particular day and at max to two.And soon or late filling of form doesn't matter as the roll numbers are awarded randomly.So, dont think too much about it and focus on studying. All the best!