According to Jones and Williams (1873), gurt is an adjective meaning ‘great’. And yes, it does, to a certain extent. But the word is more often used in an adverbial form, typically in place of ‘very’.
That ice-cream was gurt yuge
DEM NOUN VB ADV ADJ
So, again, do we consider this a shift or expansion in the use of the word, or was the interpretation by Jones and Williams not entirely thorough?
Gurt is also found in a noun form, but it isn’t directly associated with Somerset dialect. According to the Dictionary.com ‘a gurt’ is a mining term for a ‘gutter or channel hewn from the bottom of a working drift’. This, however, seems to be a US mining term, as the definition came from the Webster’s Dictionary.
I don’t believe the mining noun-form of the word has ever been in use in Somerset (do correct me if I’m wrong), and that more likely it started life as a derivative of ‘great’ before being used as an adverb. It’s still very much in use in Somerset today, and if you ask anyone from Somerset what it means, they’ll probably say ‘great’ or ‘very’, or maybe not even have a definition, just tell you how to use it. But it’s become much more than great. It’s become a word in it’s own right, rather than a synonym. And that, well that is just gurt lush!