The GATE and NET both are national level entrance examinations which are conducted for different purposes.
Gate is an Indian entrance examinations which is jointly conducted by various IITs. The main purpose of this examination is examining the ability of candidates in various undergraduate programs in Architecture/Engineering/Technology and providing admission to post graduate level in the same field.
Where as NET is conducted to determine the eligibility of science aspirant for Lectureship and for the honour of Junior research fellowship. Those who have completed graduation and post graduation are eligible to appear in the examination.
The Gate consists of Engineering mathematics where as NET deals with Mathematical Sciences. Both are two different syllabus. Hence you need to prepare different strategies for both examination.
Hope this helps !
Good luck :)