Edit: Keycloak is licensed with the Apache 2.0 license, so none of this is relevant for Keycloak.
GPL is only a problem if you import or change the source code. If you just run it in the backend, as a service, you're most likely fine.
If you customise Keycloak through code, you're probably in GPL violation territory. With the customisability of Keycloak, I doubt that this is something many projects will ever run into.
That's true; assuming you run the software on your own premises, GPL won't hurt you at all. If you sell premium software packages to be run over at your clients' hardware this can be a problem, though.
However, after looking into Keycloak more closely, the software seems to be licensed with the Apache 2 license, so none of this is a concern.
GPL is only a problem if you import or change the source code. If you just run it in the backend, as a service, you're most likely fine.
If you customise Keycloak through code, you're probably in GPL violation territory. With the customisability of Keycloak, I doubt that this is something many projects will ever run into.