This members-only tutorial provides the steps to add SVG icons before the buttons' filter text when using Buttons-type of facet with WP Grid Builder.
Create your Buttons type of facet.
In my test site I set the facet to filter by restaurant_type, a custom taxonomy.
Edit your SVG icons in a text/code editor like Sublime Text and set the fill color to #ffffff.
This is because we are going to be setting the .svg files as background images using CSS. For us to able to control the color of the stroke for regular and pressed states via filter property, a light color or white makes it easier/possible.