Gust is a developer first WordPress theme and we expect you'll want to change things. Almost everything in Gust is configurable or editable. You should find the resources you need here to get started extending Gust.
If you're extending Gust in any way, using a child theme is a must. If you're unfamiliar with child themes, essentially it allows you to "inherit" code from another theme, allowing you to make changes while also receiving updates for that theme.
We've created a starter child theme for you that you can download here.
Once you've added your child theme, make sure you activate it. You won't lose any of your configurations to Gust or any of your custom pages.
When you're using the page builder, Gust keeps track of the class names used and automatically recompiles the styles necessary. However, if you're doing any work outside of the page builder, you'll need to ensure that any class names there are also included the final CSS generated by Tailwind. Read more on the templates page.