Wordpress - Where can I find the Gutenberg block icons?

Gutenberg is making use of dashicons.
You can find your example icon here, and a cheat sheet of dashicons here.

Here you can find a list of all the Gutenberg icons with name and preview: https://wordpress.github.io/gutenberg/?path=/story/icons-icon--library

In order to use this, you'll need to copy the actual SVG source code.

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Example: register an ACF block with the postExcerpt icon

You'll need to inspect the actual element and extract the source code of the SVG icon from https://wordpress.github.io/gutenberg/?path=/story/icons-icon--library:

<svg><path d="M12.75 9.333c0 .521-.102.977-.327 1.354-.23.386-.555.628-.893.774-.545.234-1.183.227-1.544.222l-.12-.001v-1.5h.123c.414.001.715.002.948-.099a.395.395 0 00.199-.166c.05-.083.114-.253.114-.584V7.2H8.8V4h3.95v5.333zM7.95 9.333c0 .521-.102.977-.327 1.354-.23.386-.555.628-.893.774-.545.234-1.183.227-1.544.222l-.12-.001v-1.5h.123c.414.001.715.002.948-.099a.394.394 0 00.198-.166c.05-.083.115-.253.115-.584V7.2H4V4h3.95v5.333zM13 20H4v-1.5h9V20zM20 16H4v-1.5h16V16z"></path></svg>

Then, you can register your block like this:

add_action('acf/init', 'my_acf_blocks_init');
function my_acf_blocks_init() {

    // Check function exists.
    if( function_exists('acf_register_block_type') ) {

        // Register a testimonial block.
        acf_register_block_type(array(
            'name'              => 'testimonial',
            'title'             => __('Testimonial'),
            'description'       => __('A custom testimonial block.'),
            'render_template'   => 'template-parts/blocks/testimonial/testimonial.php',
            'category'          => 'formatting',
            'icon'              => '<svg><path d="M12.75 9.333c0 .521-.102.977-.327 1.354-.23.386-.555.628-.893.774-.545.234-1.183.227-1.544.222l-.12-.001v-1.5h.123c.414.001.715.002.948-.099a.395.395 0 00.199-.166c.05-.083.114-.253.114-.584V7.2H8.8V4h3.95v5.333zM7.95 9.333c0 .521-.102.977-.327 1.354-.23.386-.555.628-.893.774-.545.234-1.183.227-1.544.222l-.12-.001v-1.5h.123c.414.001.715.002.948-.099a.394.394 0 00.198-.166c.05-.083.115-.253.115-.584V7.2H4V4h3.95v5.333zM13 20H4v-1.5h9V20zM20 16H4v-1.5h16V16z"></path></svg>'
        ));
    }
}

Result:

More information about ACF blocks at https://www.advancedcustomfields.com/resources/acf_register_block_type/