CSS Grid Garden
This site offers an interactive game based way of learning grid. As the user progresses the content that they are learning becomes exponentially more complex until the game ends at level 28 and all the areas of grid have been explored and mastered.
CSS Tricks
This site offers a side by side view of the coding on the parent container and then the coding for the chilren of that container. It is a nice way of seeing the relationship between the grid container and the grid items.
Front-End Curriculum Grid Lesson
This site is set up as an educational site. It is designed to teaxh the user how to use grid. It takes the user step by step trough all the properties of grid and how they can be used.
Summary
There are numerous sites offering information about grid. The developer just needs to find the site that best fits their learning style. For example, Grid Garden might be a fun way to begin learning, but not practical for looking up how to use a property and value. Front-End is and educational site specifically geared towards learning grid. Where as CSS Tricks might be good when thinking about the relationship of grid items to the parent container.