"goirt" versus "ghoirt" and recording
There are individual recordings for "goirt" and "ghoirt" that seem really similar to me - I can't tell the difference. When the question is to identify a single word for one of these I'm always guessing. In a sentence the other words are a clue, but when it is only to identify the individual word I never know which to choose.
It's definitely something of a milder difference in the example you're talking about. He's quite gentle on the ‘gh’. It's good that it's the same speaker in this case though—makes it a touch harder 'cause you can't just match the voice, but also you can hear the difference within one voice rather than between potentially different accents/dialects.
Can you reproduce the sound of the ‘gh’ yourself? I find I get better at differentiating sounds I'm not used to by getting better at pronouncing them. The broad ‘gh’ (and ‘dh’ too) is pronounced in a way not too dissimilar from the English hard ‘g’ (like in ‘good’), but instead of being a full stopped closure like ‘g’ is, it's a constriction that never quite closes.
It's a similar sort of sound to ‘v’ or ‘z’ (be sure to think of their sounds, not the letter names), in that if you think about how they're made, they're constrictions at different points of your mouth that aren't quite a full closure, making a voiced buzzing sort of sound. ‘Gh’ is the same thing made at the point of contact where you'd say ‘g’.
Incidentally, if you're Scottish, and you can pronounce the ‘ch’ in ‘loch’ correctly, an easier way to find the sound might be to think of it like this—it's the ‘ch’ sound but with your voice added, much like how ‘z’ is ‘s’ with your voice added, or ‘v’ is ‘f’ with your voice added.
I know this doesn't exactly address your post directly, but it's the number one solution to differentiating similar sounds IMO. Especially when one sound is non-native to us, our ears seem to like to map them to similar sounds that are native to us.
In the example you mentioned, the hard stop of the ‘g’ isn't quite there in the ‘ghoirt’ clip, which is really the best giveaway. It's more like he's gliding past a ‘g’ rather than pronouncing it—as it happens, this is how Spanish treats the letter ‘g’ between vowels, softly gliding over it rather than enunciating it like a hard ‘g’. It results in a somewhat similar (or same, depending on the dialect) sound to the Gaelic ‘gh’/‘dh’.
Hopefully that's of some use. :)
Thank you for such a long and detailed answer! Now I will try to find these recordings again to see if I can hear the difference now. Edit - it is harder than I thought to get a specific question to come up again! I've done several review practice quizzes now. I'll just have to keep watching for it I guess. Thanks again.