Can I cluster the the API Gateway and have the load balancer in front of it.
Yes, you can. Most of the good Api Gateway solutions will provide the ability to do clustering. e.g. https://getkong.org/docs/0.9.x/clustering/ or you can use cloud based Api Gateway: Azure API Management or AWS API Gateway
What do I need to consider with respect to managing authentication?
These specifics depends on your selection of API Gateway solution.