How do I add custom CSS animations for specific Gutenberg blocks?

Many blocks allow you to add a class and/or an ID. If you select the block in the Editor, then look on the right side of the screen under the Advanced tab, it will often show “HTML Anchor” (which is an ID) and “Additional CSS class(es)”. If the effect is something you plan to repeat, a class is often a good way to go – say, word-animation or something descriptive enough that it’s not likely to conflict with other CSS on your website.

As to how to add the CSS itself, there are a variety of ways. Many themes allow you to go to Appearance > Customize, then into an Additional CSS area, and you can add your CSS there. If your theme doesn’t support that, you can either look into a plugin that allows you to add in your custom CSS, or else create a child theme. The child theme doesn’t require a lot of code but can sometimes feel more intimidating than the Customizer or a plugin that just lets you write it in.