The IG is highly available by design, you don't have to worry about it, it's a logical device that is handled by multiple physical networking devices - you don't have visability of them and you dont need it.
As Adrian said, the IGW "just works." To expand on this a bit, we know that it will handle whatever traffic it receives because it falls under the AWS global infrastructure section of the shared responsibility model. So as an AWS customer, you can rely on Amazon to handle this service for you. For more information: https://aws.amazon.com/compliance/shared-responsibility-model/