Glassfish Application Server has been the reference implementation of Java EE since Java EE 5. Those days, it was Glassfish version 2. Java EE 6 reference implementation was Glassfish 3.x, Java EE 7 reference implementation was Glassfish 4.0, Java EE 8 reference implementation was Glassfish 5.0.
A little bit over a year ago it was announced that Java EE is to be contributed to the Eclipse Foundation. The process was not a trivial task, the code to be contributed was revisioned and updated to be legally correct, secure, and cleaned. Right before each project on GitHub was archived and transferred under Eclipse Foundation and Jakarta EE, one last version of it was released. This is what makes Oracle Glassfish 5.0.1, the latest reference implementation of Java EE 8.
Eclipse Glassfish 5.1.0, on the other hand, is Jakarta EE implementation. It is the initial Jakarta EE release, that has a binary compatible API with Java EE and Java EE Compatibility Test Suite should pass with this initial Jakarta EE release. There is some difference between Java EE 8 and Jakarta EE, though.
The contributed Java EE projects were subject of community revision. While only small changes that do not change the API were allowed for the initial release, the projects updated their dependencies, updated functionality to work with them, fixed some possible bugs, polished documentation, and javadoc, and adopted the Eclipse license.
After the hard work that started at the end of 2017 by Oracle contributing millions of rows of Java EE code, and continued by the Eclipse Foundation and the community, after a little more than a year, January 29 is to be the Eclipse Glassfish 5.1.0, the Jakarta EE implementation, release date.