how rank is calculated in GATE?

No semester marks have no impact in the evaluation of Gate rank. Your Gate rank is calculated from your Gate exam marks. For understanding this ,I think you should be clear about three terms :  Gate Rank, Gate Score and Gate Percentile.
Gate Rank : GATE Rank also called as AIR or All India Rank tells how far are you from top in a particular year and in a particular subject. Every GATE Qualified Candidate is given a GATE Rank. Its an integer number which tells your position in the exam and paper for the year in which you have given the exam. Example AIR-40 in CS-16 means you are at the 40th position from top in Computer Science GATE Exam conducted in year 2016.
Gate Score :GATE score is calculated out of 1000, unlike marks which are out of 100. It is calculated using a formula which can be found on organizing institute's website. In fact, this is the most important term used for admission and in declaring cut off s for call for Interview/Test and admission.
Gate Percentile:It is the percentage of people behind you (as per rank) in gate exam. The lower your rank is, higher will be the percentile. Toppers percentile is 100 and last rank holders percentile is 0.
Most of the colleges release their MTech cut off list mentioning Gate Scores.

Yes , previous year questions some time repeat . But if you solve and practice previous 15 years Gate papers , you will be able to tackle almost all questions in your Gate exam. Because practicing previous year question papers will give you an idea regarding the type of questions that can come in your Gate exam.
Make sure you are elevating the level of your conceptual clarity by solving questions.
Also , be ready for new and unexpected questions in your gate exam because for sure all the questions are not repeated and cannot be of same type.