hi,

see there is no specific marks is applied for each of the subjects for gate exam, however you can expect 15 marks of general aptitude questions and 10-15 marks of engineering mathematics questions, however for other subjects there is no specific marks in prescribed by iits, I suggest you to check the previous years papers to learn about most important subjects to follow, you may follow the below link to know the syllabus of gate civil engineering,

https://engineering.careers360.com/articles/gate-syllabus-for-civil-engineering