First i want to let you knw tht scoring 100 is very diificult even 90 also is very difficult to score .

for gate you must have a degree of btech/BE/bsc. you can start preparing for gate from 1 year only there ia limit for attending the gate exam nut there is limit in preparing for gate exam.

you can start preparing engg. mathematics `from your first or second year cuz it is a part of first or second year.start preparing as soon as possible go for good books or best books consult teachers if u want cuz then only u can clear it and aptitude is also in the syllabus solve papers for aptitude ..

BEST OF LUCK:)