GATE

Hello Ajinkya,
Ideally, one should start preparing from the third year of engineering. GATE syllabus comprises of the same syllabus as of engineering. By the commencement of the third year of engineering, you will already be familiar with a major part of the syllabus. Therefore, you can start your preparation from the third year of engineering. It is recommended to opt for a GATE coaching institute as they help to provide proper guidance. They conduct periodic tests to help you realise your string and weak sections which will help you figure out where you need to put more focus on.
I hope this answers your question to your satisfaction. If so, kindly upvote this answer. If you have any further questions, kindly revert.

There are opportunities that you do not need a GATE score to get into an M.Tech course in Institutes like IIT and NIT.
If you are a student of a central-funded university having CGPA of 8. Then, there is no need of GATE score for you. You can avail direct interview for M.Tech in IITs.
If you are among top 10 students at university level.no need for GATE score.
If you do have teaching (or any other in the field) experience of 2 years after B.Tech, then also no need for GATE score even for M.Tech/PhD/MS(research)programs in IITs.
If you are a sponsored candidate (i.e if your employer sponsors your education) no need of GATE score for you.
Some options for you:
M.Tech/ ME in Design/ Fluid Mechanics/ Gas Dynamics/ Production/ Robotics/ Mechatronics/ CAD-CAE etc.
PhD in Mechanical Engineering followed by post-doctoral studies
PGDBM/MBA - from Ivy League colleges
M.Sc in statistics
Any certificate course in Quality Management
Learn a few software like Ansys, Fluent, CFX, SolidWorks etc.
There are many good institutes which conduct their own exam such as BITS, VIT etc.

Hello Waqar,
The cut-off for GATE is 25 for 2017. Therefore, you are not eligible to appear for selection based on GATE scores. However, you can apply for various state level entrance examinations like OJEE for Odisha, WBUT for West Bengal, PGECET for Andhra Pradesh, Telangana, Karnataka etc. Based on these scores, you can choose a college for yourself for M.Tech. Apart from this, some private universities like GITAM (Vishakhapatnam), SRM University, KIIT (Odisha), VIT University, AMITY etc. conduct their own entrance examination. You can apply for these as well.
I hope this answers your question. If you have any further questions, kindly revert.