I am pasting here the answer you got in the private ticket so that the community can benefit from it too.
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Generally speaking, the 'GridCommandButton' 'Icon' parameter modifies the content of the button, rendering an SVG tag (icon) within the <button> tag. However, if you want to include your custom icon, you can declare it using a <img> or <svg> tag directly within the 'GridCommandButton' body.