WordPress at a glance
action is not described

woocommerce_api_wc_gateway_paypal action-hook . WC 1.0

Usage

add_action( 'woocommerce_api_wc_gateway_paypal', 'action_function_name_3629' );
function action_function_name_3629(){
	// action...
}

Where the hook is called

woocommerce_legacy_paypal_ipn()
woocommerce_api_wc_gateway_paypal
woocommerce/includes/wc-deprecated-functions.php 226
do_action( 'woocommerce_api_wc_gateway_paypal' );

Where in WP core the hook is used WooCommerce

woocommerce/includes/gateways/paypal/includes/class-wc-gateway-paypal-ipn-handler.php 34
add_action( 'woocommerce_api_wc_gateway_paypal', array( $this, 'check_response' ) );