The list and configuration details of currently out-of-the-box available Notification sending gateways are here: https://virtocommerce.com/docs/vc2devguide/deployment/platform-settings.
A new gateway should be implemented in a custom module (and registered in DI). E.g., https://github.com/VirtoCommerce/vc-platform/blob/master/VirtoCommerce.Platform.Data.Notifications/SendGridEmailNotificationSendingGateway.cs
Also, check WhatsApp notification gateway sample https://github.com/VirtoCommerce/vc-samples/tree/master/WhatsAppNotification