Do not mug up Maths by going through whole textbooks/standard books, it is waste of time and moreover, if you try to learn, you won’t finish learning it even after the GATE exam is over.
However, Candidate can refer standard book such as Higher Engineering Mathematics by B.S. Grewal and advanced engineering mathematics by Erwin kreyszig for noting down important formulae and shortcuts.
Study the list of topics on the priority basis and practice previous year problems after completion of each topic. This process would give you familiarity with level and pattern of questions.