In the sense of GPL, is a Web application running on a private server without exposing its code considered "released"? (or is the word "distributed"?)
Forget for a second about the GPL and licenses entirely and let me focus on the parts of a typical web app: you will have a backend and a frontend.
The backend may connect to some database, do some heavy processing, business logic, etc. It may generate dynamically web pages.
The frontend is whatever ends up being downloaded in a user's web browser.
Let's assume also that you are running this server privately and exposing the web application on the public internet. The private server for private use is not interesting here as no external redistribution is taking place.
Now in this context, what software is being redistributed?
Therefore, if a GPL component is used in a Web application running on a private server without exposing its code, does this make the copyright holder of the application obliged to release its source?
Based on the context I outlined above, things become much clearer:
In most cases, the frontend code is redistributed and the GPL applies.
In some cases -and this is a grey'ish area- some backend corresponding source code may be redistributable as the corresponding source code for the frontend.