WP Query: orderby with one meta key, but multiple values

Use this code instead-

$supplierArgs = array(
    'post_type' => 'service_providers',
    'meta_query' => array(
        'relation' => 'OR',
        array(
            'key'     => 'service_provider_tier',
            'value'   => 'G',
            'compare' => '='
        ),
        array(
            'key'     => 'service_provider_tier',
            'value'   => 'S',
            'compare' => '='
        ),
        array(
            'key'     => 'service_provider_tier',
            'value'   => 'B',
            'compare' => '='
        ),
    ),
    'orderby' => 'meta_value',
    'meta_key' => 'service_provider_tier',
    'meta_type' => 'CHAR',
    'order' => 'ASC'
);

$query = new WP_Query( $supplierArgs );

This approach uses meta_value to order the posts and assigns the custom field service_provider_tier to meta_key. It orders the results by the value of the service_provider_tier meta key. If you need a custom order (Gold, Silver, Basic), you may have to manipulate the query results after fetching them or use a custom sorting function.