This CSS Gradient Generator by Josh W Comeau is a very good, simple Web App that helps you to create "Beautiful, lush gradients".
I personally struggle mapping the syntax to a visual so this tool has been a great help for me. It focuses on creating linear gradients and it allows you to configure them using a number of different color modes (like HCL) even though they aren't directly supported on the
linear-gradient CSS function. Josh's supporting blog post goes into more detail about how the color interpolation is calculated and applied to something the browser can render—it's a fascinating read into some of the theory behind how it works.