I learn Gatling using the resources I found online and experimentation. Just google "Gatling tutorial", there are plenty of links to pages showing Gatling basics, many focus on different aspects so it's useful to read a number of them. The [Gatling documentation 3.0][1] is pretty good as well, they also have an [Advanced Tutorial][2] covering more stuff. Gatling is open source, so you can always browse the source if you need more clarity on how something works.

For High-Level understanding, you may start with video
https://www.youtube.com/watch?v=VUPTaPms210


  [1]: https://gatling.io/docs/3.0/
  [2]: https://gatling.io/docs/current/advanced_tutorial/