In a way, yes, GlassFish is a Java framework. Although also much more.

It allows running and manage applications on a server, which provides resources to the applications. And it also allows building standalone Java applications, with the server embedded in it, in a way that you would expect from a framework.

On top of that, it provides standard Jakarta EE APIs, so your applications don't need GlassFish, you can run them on other servers too. Or you can easily migrte from other servers and frameworks to GlassFish if you like it more. And you can learn Jakarta EE APIs even before you will use GlassFish, or hire somebody who already knows it even though they never used GlassFish.

er... surely it's a Java application server?

(i.e. in the same space as Jboss/Wildfly, WebSphere, etc)

Historically, it was also the reference implementation application server for J2EE.

Nope. From the glassfish.org web page: "Eclipse GlassFish is a lightweight yet powerful open-source application server that fully implements the Jakarta EE platform."