Recently I took Gmat and received 710 points, which is a relatively decent score and it was sufficient to get me into one of the top business schools in Europe. I was able to prepare for GMAT in 3 months and got the desired score in my first attempt. The mock exams were 690 and 720.
I actually made a video on that so feel free to check it out

The link:
But in short:
RESOURCES THAT I USED:
1.Official guides
2.Sentence Correction Megathread
3.RC Megathread
4.Quant Megathreadl
5. Youtube channels:
Gmatclub
Veritas Prep
PrepScholarGMAT
Dominate the GMAT
6. Some really good Youtube videos:
How to become your own GMAT tutor: GMAT Ninja LIVE Series 2, Video 1
GMAT Math Review Part 1 (by Neeley TCU)
7.The “FREE STUFF” (free trials):
EGMAT
Economist GMAT
8.
Manhattan Prep GMAT Blog
9. Manhattan SC book
Insights and mistakes
The first one is that every expert out there tries to differentiate and has his own special system or strategy on how to solve gmat problems and initially, i was rather overwhelmed by all those strategies. However, during studies, i found out that all of those strategies, while they have different names they are based on the same and the main principles. The main ideas that were truly helpful for me were:
Process of elimination
Ballparking
Reading the question or the sentence carefully
Plugging in some numbers
Thinking like a test maker
Knowing when and how to make an educated guess
Tracking ones time and cutting losses if needed
All of these have truly helped me during the exam and I recommend looking into them.
Furthermore, initially, i have made a huge mistake and solved questions without properly reflecting on them. Most of your time should be spent not on solving questions, but on reflecting on your mistakes and making sure that you do not make them again!
Lastly, do not get discouraged if you are not making any progress, there are a lot of success stories on the internet, however, in my opinion, they do not reflect reality at all - preparing for gmat is not easy, it requires time and dedication. Probably the main reason why I was able to prepare in three months was that I already had a decent knowledge of math and english even before starting to prepare.
BIG THANKS to GMAT NINJA! And GMAT club community overall !!!