It just goes to show how much of the content of the Internet is just bollox, doesn't it!
Other than having sung the Somerset Wassail in the past, I do NOT claim to have any actual verifiable information concerning its origins of meanings, but seriously doubt that Eric Von Daniken's aliens have anything to do with a Wassail. They may have built runways for spaceships on the plains of Nazcar in South America. but I doubt that as an encore they went on to sculpt the river Parrett.

As for a Zodiac "discovered" in 1930 - didn't the ancient Egyptians, the Romans, The Greeks and the Norse already have a zodiac. And anyway the song pre-dates 1930, so that reference can be binned.
I am also wary of anything quoted about King Arthur because most of it is made up to suit the teller. Especially stuff on the internet, which is a haven for fantasists and so-called New Age mumbo-jumbo.

As for spelling, I would reckon that the word "Gurt" is used as a corruption/dialect for "Great" as in "Big" whereas "Girt" is derived from "Girdle" and in different usages can be the past tense of "Gird" i.e. to put a belt round, or to encirle , or to measure the girth of... (actually "Gurt" is a mining term for a gutter or water channel - so we can discount that usage).
Searching for the "Gurt dog of Langport" brings up a more believable explanation Black Dog - Canine Apparitions Revisited and Beyond - the song is referred to at the very bottom of the page. and reckons :-
Secondly the Vikings were likened to wolves themselves. There is an old Somerset wassailing song whose chorus goes;

The Black Dog of Langport have a-burned off his tail;
And this is the night of our jolly wassail;
Vor tis our wassail,
And tis your wassial,
And joy be to you, vor tis our wassail.

It is said that The Black Dog is a reference to the defeat of the Danes by King Alfred in 878.


As I say - I have NO verifiable references, and no particular axe to grind. I am just sceptical of some of the outrageous claims and statements on the Wibbly Wobbly Web.
Best of luck with the search.
Quack!!
Geoff the Duck.