If you want to remove gender field which is available in traveler form in checkout page, please add following given code in functions.php file of your child theme.

function wp_travel_remove_gender( $checkout_fields ){

         unset( $checkout_fields['traveller_fields']['gender'] );
          return $checkout_fields;
 }

add_filter('wp_travel_checkout_fields', 'wp_travel_remove_gender' );

Before:

After removing field:

Also, regarding creating child theme, please refer link.

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