Is there any api integration for gcash payment?

Not sure if GCash provides their own API but Paymongo has an API that can collect G-Cash payments. Perhaps you can check it out. Looks like they just charge per transaction.

Per https://developers.paymongo.com/docs/accepting-gcash-payments, Collecting GCash payments on your website starts with creating a resource to generate a checkout URL where your customer needs to authorize an amount, wait for the authorization to complete and create a Payment resource to receive the authorized amount. A Source resource is used to generate GCash checkout URL to authorize a certain amount to be deducted from your customer's GCash account and send it to your PayMongo account. After completing the authorization, your integration uses the chargeable source to make a create payment request and receive the payment.

Specific API endpoints are listed on https://developers.paymongo.com/reference.

You can try using curl. This link may help you.

curl https://checkout-test.adyen.com/v66/payments \
-H "x-API-key: YOUR_X-API-KEY" \
-H "content-type: application/json" \
-d '{
  "merchantAccount":"YOUR_MERCHANT_ACCOUNT",
  "reference":"YOUR_ORDER_NUMBER",
  "amount":{
    "currency":"PHP",
    "value":1000
  },
  "paymentMethod":{
    "type":"gcash"
  },
  "returnUrl":"https://your-company.com/checkout?shopperOrder=12xy.."
}'

Adyen PHP G-Cash

$client = new \Adyen\Client();
$client->setXApiKey("YOUR_X-API-KEY");
$service = new \Adyen\Service\Checkout($client);
 
$params = array(
  "amount" => array(
    "currency" => "PHP",
    "value" => 1000
  ),
  "reference" => "YOUR_ORDER_NUMBER",
  "paymentMethod" => array(
    "type" => "gcash"
  ),
  "returnUrl" => "https://your-company.com/checkout?shopperOrder=12xy..",
  "merchantAccount" => "YOUR_MERCHANT_ACCOUNT"
);
$result = $service->payments($params);