I've used this tag a few times and I find it personally useful. As you pointed out, there are quite a few questions that would probably be asking for a GUI. This isn't how I think the tag should be used, though. Personally, I feel the tag should be used on a question where a Command Line Interface (CLI) software could conceivably be answered. If the OP doesn't want a CLI, then the tag gui should be used.
For example, if I were to hypothetically ask the question:
Tool to easily list and edit cron jobs on Linux
Alright, not the best question, but I trying to make a point here. A CLI could be given as an answer but what if I'm looking for a GUI instead of a CLI? This is where I would use the tag, gui. Same with CLI, if I wanted a CLI and not a GUI, I would use the tag cli.
I don't think we should ever assume a specification for a question. I also think we should keep the tag gui and should be used in the case that without the tag on a question, the question could then be answered by CLI software answers.