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. TheI always found official resource helpful, GatlingThe Gatling documentation 3.0 is pretty good as well, they also have an Advanced Tutorial 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