The Vocabulary of Sir Gawain and the Green Knight
My comments on the vocabulary of Sir Gawain and the Green Knight fall into two categories: literary significance and readability.
Literary Significance
A rough breakdown of the origins of the words in the poem reveal that, although about 60% of the words come from Old English, just over a quarter come from French and just under 10% from Old Norse. These numbers are not surprising; many loanwords from French and Old Norse were in common use all over the country by the fourteenth century. However, the dialect of the Gawain-poet is notable for its high proportion of words of Old Norse origin which were not widespread. Such words are often found in alliterating syllables, suggesting that they were in part a poetic, rather than a regional vocabulary (though the two are not always mutually exclusive). The Scandinavian element in the language is not the only conspicuous feature in this capacity: numerous words of Old English origin which were no longer used appear in the poem, perhaps as 'poetic archaisms'. It may be possible to talk about a regional poetic vocabulary in the poem, which may affect how we interpret the poem's themes.
It is also noteworthy that, enough of the French vocabulary in the poem is of a technical or current nature to suggest that the poet was fully informed of the wider literary culture of the England of his day. Of course, this type of statement is supported by literary, as well as linguistic evidence.
Readability
The greatest barrier to the readability of the poem for modern students is the archaic, dialectal, or poetic vocabulary (however you class it) which is not in the standard language of today. In some parts of the poem there is also use of heavy technical jargon, such as the hunt scene (how many of you know the technical terms, even in Modern English, for how to butcher a deer?). We are forced to rely on the glossary for this. I recommend taking some conscious step to learn the strange vocabulary, such as writing the meanings of the words in the margin or making flashcards. You want to avoid looking up the same word multiple times, if possible.
You will inevitably find yourself looking up words you don't recognise, only to find that they are average, non-technical, non-archaic words you use every day, and that you have simply been fooled by the spelling. If this makes you feel stupid, channel your humiliation into learning the spelling system better. The problem will go away, and you'll end up spending far less time going to the back of the book.
That's it! You're done! Go read (and enjoy) the poem....