Is it possible to clear the GATE without any coaching (by self-studying)?
Yes. Very much possible!
Earlier times(say 6–7 years ago) there was a huge gap between college education and the requirements of GATE exam and coaching institutions bridged that gap. So only a handful of financially strong students got the quality content from those coachings and cracked the exam with ease.
Cut to 2019. That gap is being bridged by YouTube. Now with a little effort, you can find quality contents in YouTube for free which has increased competition significantly. If you see Marks vs Rank analysis of GATE 19 EE, you will see that almost 90 students crossed 90 marks out of 100. So to get a good rank, just collecting good content isn't enough because everyone has the same content now. So you have to put extra effort for smart planning and execution. Make a long term plan (monthly basis, like you will finish X,Y,Z subjects in next 2 months) and a short term plan (daily basis like today you will complete A,B,C topic and solve 40 problems from topic A).
Make one short note for all subjects in one notebook of around 100 page consisting of formulas with brief descriptions, diagrams, important conclusions and leave a page blank for every subjects in the notebook. Later you will write down your mistakes in that blank page and keep updating the short note.
Purchase 3 test series as early as you can. I would recommend Kreatryx, Madeeasy and Ace Academy test series. Don't waste time comparing which test series is good and which is bad. Every test series will have some really good question as well as some irrelevant questions. Don't skip any tests. Start from Topicwise to subject wise to Full length tests. Try completing all subject wise tests by the end of November. Give around 35 full length tests in the last 2 months. Don't get demoralized if you get low marks in any test. Analyze your mistakes and be happy that you learned something from that test. If you keep doing same type of mistakes,then add it to your short notes mistakes page. Keep improving till the last test.
If you keep improving continuously everyday, and track your progress (which will motivate you in the meantime), Nobody can stop you from achieving your dreams.
Best of Luck.
Here's a list of materials that you can follow if you are from EE:
Machines: Varun D Nair and NPTEL videos.
Analog, Digital, Signal, Control: search ‘GATE CRACKERS' channel / Geniqe Education
Mathematics: IIT JEE coaching centers videos
Aptitude: search ‘Takshzilla' on YouTube
Power system: Genique education or
Power electronics: search GATEMATIC channel
...And the number of content is increasing very fast everyday. So keep searching for other subjects as well.