Gave gate twice. In 2018 - 51 marks. In 2019 - 59 marks. Here's my list (including mistakes I did for Gate in general) -
1. There was a fragmentation question in gate 18. I forgot to divide by 8 to calculate offset. In Gate 19, I forgot to multiply by 8 (8 word block size) in a question where max memory bandwidth was asked. So, missing your answers by one inch.
2. Assuming question is difficult and leaving without even reading it properly. In Gate 19, I left room allocation question without even reading it as i thought it is difficult. Later realized it wouldn't have took more than two minutes. It was worth two marks.
3. There was sql question in Gate 19. According to sql query we were supposed to do only cross product as there was no join condition but we tend to do what we have done a thousand time ignoring the question requires something bit different. I did natural join. Again 2 marks.
4. Trying to predict the difficulty level of exam in advance and calculating how much marks do you need and similar kind of things. I told myself entire year that I need 60+ marks. I got 59, close to what I wanted. But, as you see in Gate 19 that's just not enough. Aim for maximum marks only.
5. Leaving difficult topics is another mistake people do and I have done too both the times. You just can't do that. I left engineering maths both the times and it cost me at least 3-4 marks that I would have scored easily if I hadn't left it.
7. Aptitude and maths is ultimately what will make you topper. You can't run away from that. You need to score close to 100 percent in aptitude. You need to be really good in maths as probability can pop up in any of the 65 questions, gate 19 is a good example of that. For discrete maths, you must read kenneth rosen and solve exercises. Notes won't help you for the kind of question that come in discrete in gate.
7. For 2nd timers - Don't repeat what you have done first time for preparation. I did that. It won't get you anywhere. Just because it's your 2nd attempt doesn't mean you'll get better rank. You need to level up and work really hard to improve.
8. Lastly, you need to push and do what's hard. Everyone is doing the easy stuff. You can't score high and be in your comfort zone at the same time.
Other mistakes that you usually do in test like bit-bytes, missing correct - incorrect, true - false kind of things can be fixed if you give a large number of test. I gave 80 test and these were reduced greatly. Aim for 100+ test. Same for negative marking. Both the times I got only 1 mark negative.
Good luck :)