With the next request, you can get all info about a specific payment gateway:

method: GET

URL: /management/payment_gateways/[PAYMENT_GATEWAY_ID].json

RETURN:
{
  "page": "payment_gateway",
  "payment_gateway": {
      "name": "NAME",
      "provider": "PROVIDER",
      "public_api_key": "PUBLIC API KEY",
      "enabled": true/false,
      "test_mode": true/false
  },
  "pagination": {
  "current": NUMBER,
  "last": true/false
  }
}