Change 'gravity-forms' to whatever is shown in the URL for the Gravity Forms page. Example "options-general.php?page=gravity-forms"

hook-css.php

<?php

function hook_my_css() {
  $screen = get_current_screen();
  if ('gravity-forms' == $screen->id ) {
    wp_enqueue_style( 'my-css', plugins_url( '/my.css', __FILE__ ) );
  }
}

add_action( 'admin_print_scripts', 'hook_my_css' );