Somehow on one of my development boxes, the default identity provider got named default_0 and I decided to change it’s name to default. There were no other identity providers configured.
Making this change effectively prevented me from logging into the gateway. The gateway web page would not even let me type in a user id.
After restoring the gateway backup, I started the process again. This time I took a look at the Security/General page both before an after renaming the identity provider and noticed that the System Identity provider was blank after renaming.
I imagine that there is a way to fix mistake without restoring a gateway backup, but I am very glad that I was working on a development system, not a production system.