When learning Grails or any other framework, I recommend to study each components separately first. Rather than tackling multiple topics at the same time. For Grails beginners, I recommend to start with controllers. This is one of the simplest to understand and you can see results right away!
Grails is a powerful framework that makes most programming tasks short and simple. A key ingredient to achieving this is the powerful and cool language called Groovy. Before diving into learning Grails, it is worthwhile to learn basic Groovy coding tricks.
Before going through your journey in learning Grails, it is important to understand the MVC Pattern. Most web application framework uses this architecture to help make clean and manageable projects. It is surprisingly simple and easy to learn too.