The generate_svg_icon filter allows you to overwrite the entire HTML output of the icon, including the surrounding gp_icon element.
For example, if you want to change the default menu cart icon to a custom icon, then this snippet can be used:
add_filter( 'generate_svg_icon', function( $output, $icon ) {
if ( 'shopping-cart' === $icon ) {
$svg = '<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" fill="currentColor" class="bi bi-alarm-fill" viewBox="0 0 16 16">
<path d="M6 .5a.5.5 0 0 1 .5-.5h3a.5.5 0 0 1 0 1H9v1.07a7.001 7.001 0 0 1 3.274 12.474l.601.602a.5.5 0 0 1-.707.708l-.746-.746A6.97 6.97 0 0 1 8 16a6.97 6.97 0 0 1-3.422-.892l-.746.746a.5.5 0 0 1-.707-.708l.602-.602A7.001 7.001 0 0 1 7 2.07V1h-.5A.5.5 0 0 1 6 .5zm2.5 5a.5.5 0 0 0-1 0v3.362l-1.429 2.38a.5.5 0 1 0 .858.515l1.5-2.5A.5.5 0 0 0 8.5 9V5.5zM.86 5.387A2.5 2.5 0 1 1 4.387 1.86 8.035 8.035 0 0 0 .86 5.387zM11.613 1.86a2.5 2.5 0 1 1 3.527 3.527 8.035 8.035 0 0 0-3.527-3.527z"/>
</svg>';
return sprintf(
'<span class="gp-icon %1$s">
%2$s
</span>',
$icon,
$svg
);
}
return $output;
}, 15, 2 );