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Really? I can automate a pipeline of GUIs? Please show me how I can automatically query a database, take that output, run complex data aggregation over those results combined with past results, send that to a remote server for storage, and email myself the result of the upload, using only a combination of GUIs.Yes, you can, and we have been doing it for decades, e.g. Automator on OS X.
Especially stuff like this is quite easily achievable: "query a database, take that output, run complex data aggregation over those results combined with past results, send that to a remote server for storage, and email myself the result of the upload, using only a combination of GUIs".
Not only that, but pipeline steps can trivially include "run a specific cli program with this as an input and pass the output to another program" (where the previous/after program can be GUI or CLI or a remote service, etc).
Nothing precludes a program with a GUI from taking formal parameters (in whatever format) and being scriptable and pipelined to other such programs.
Whether your question is if this is true for a particular program P, it might, or it might not. That's no different from command line programs which might or might not be pipeline-able between them without intermediaries doing custom scraping, parsing and massaging.
Other scriptable solutions for GUI programs but without GUI-based visual mixing and matching of pipelines include Applescript (and it's Javascript incarnation), Microsoft PowerShell, etc. This are textual based, but not CLI -- you can run them from an IDE for all the extra power that gives.
Also macro recording/playing (available in lots of GUI apps) means you don't have to write anything like "click window at pixel x,y".
And of course in the trivial case, a mice GUI can run any terminal emulator too -- making it a superset of CLI, since the GUI terminal can also include buttons, menus, show images, etc -- which a command line interface cannot -- so it gets the best of both worlds.