What extra features does Pivotal GemFire offer over Apache Geode?
From what I can see, I think the enterprise support is the only extra feature.
Yes, you are largely correct... Apache Geode and Pivotal GemFire are essentially 1 for 1 in feature sets.
This is especially true from a Spring perspective (Spring Data for Apache Geode & Pivotal GemFire (SDG and SDG ;-), Spring Session for Apache Geode & Pivotal GemFire (SSDG) and Spring Boot for Apache Geode & Pivotal GemFire (SBDG).
While Spring Data Geode and Spring Data GemFire are 2 separate GitHub Repositories up to Spring Data Moore (2.2), see here and here, that will be changing as of Spring Data Neumann (2.3) where Spring Data GemFire will become (and already has) a module (spring-data-gemfire) of the Spring Data Geode GitHub Repository; see here. This is possible because the codebases are effectively the same and because GemFire/Geode feature sets have parity.
Most of what is Pivotal GemFire today has been rolled into a platform package offered by Pivotal/VMWare in (Pivotal) CloudFoundry, known as Pivotal Cloud Cache. The platform offering certainly simplifies "running" and "managing" Apache Geode in a highly distributed and clustered enterprise environment from an "operational" perspective particularly since running this product at scale, with consideration to consistency, latency, high availability (redundancy), sizing requirements is no trivial matter.
This same Cloud Cache offering is being expanded to be run on Kubernetes (K8S) as part of the new VMWare Tanzu vision. Therefore, what we know as Pivotal GemFire today is essentially going to disappear and be replaced by Cloud Cache for CloudFoundry and K8S.
This is not to say that the commercial offering won't add additional "enterprise" class features to distinguish it from the open source offering that is Apache Geode, but I really cannot speak to that at this point in time.
Either way, you can rest assured that all things Spring for Apache Geode and Cloud Cache will 1) have you covered and 2) do so in a consistent, non-invasive manner so that you do not have to change a single line of configuration or code.
See here for our promise to users.
This also means that Spring Boot is simply the best way to integrate and build with either Apache Geode or (Pivotal/VMWare Tanzu) Cloud Cache.
Hope this helps!