'Goirt' isn't directly describing the 'stamag'. It's the difference between saying you have a sore stomach and saying your stomach is sore.
- sore stomach > stamag ghoirt
- my stomach is sore > tha mo stamag goirt
If you said 'tha mo stamag ghoirt', that would translate as 'my sore stomach is'. It is an unfinished sentence. :)
That is a logically possible interpretation and there could be some languages that do that. After all, it is not easy to tell the difference between the functions of the two wells in I sleep well so I am well, but in general, in both Gaelic and English, we use adjectives rather than adverbs in this situation. We say I am cold, rather than I am coldly.
What Joanne is getting at is that adjectives are used in two different ways, which have fancy names:
- Attributive: tha mo stamag ghoirt mì-ghoireasach - 'my sore stomach is inconvenient'
- Predicative: tha mo stamag goirt - 'my stomach is sore'
I have given a description of the difference on another question:
It is mentioned in the notes but it is not very noticeable. If you want a longer explanation I have given one answering https://forum.duolingo.com/comment/36917434?comment_id=37901499