The poem Sir Gawain and the Green Knight has survived in a single vellum manuscript, dated around 1400, and is now preserved in the British Library and on permanent display.[1]  The poem is the last of four in the re-binding of 1964.  The poem was first printed in 1839 by Sir F. Madden for the Bannatyne Club and was reproduced in facsimile by Sir I. Gollancz for the Early English Text Society in 1923.[5].  The edition by Tolkien[2] was the first modern transcription (and well before his stay at Stonyhurst College in Bowland.)

[1] The Gawain-Poet. The Cotton Nero A.x, Art. 3 Manuscript. (ca. 1400). British Library. The only surviving copy of the work of the Gawain-Poet.

[2] J R R Tolkien, E V Gordon, and Norman Davis. Sir Gawain and the Green Knight. (1967). Clarendon Press. Oxford. The first edition of 1925 by Tolkien and Gordon was re-edited by Davis in 1967 (second edition) and reprinted with corrections in 1968.  The book contains extensive notes, an appendix on the language, its grammatical structure and its pronunciation, and a comprehensive glossary of the Middle English words used in the text.

[3] James Winny. Sir Gawain and the Green Knight. Middle English Text with Facing Translation. (1992). Broadview Press. PO Box 1243, Peterborough, Ontario, Canada K9J 7H5. Middle English text, but with Middle English characters (yogh and thorn) replaced by modern English close equivalents.  This is a good, but not literal literal translation;  unfortunately almost all the alliteration of the original is lost in translation.  Still, the best introduction to the text of this major English classic literature, but try to read the original.

[4] Malcolm Andrew and Ronald Waldron. The Poems of the Pearl Manuscript. Pearl, Cleanness, Patience, Sir Gawain and the Green Knight. (1978). University of Exeter Press. The latest and definitive transcription of the text in the manuscript with many notes.  No modern English, but an extensive word glossary.  Also includes the other poems from the Nero MS.

[5] Sir. Israel Gollancz. Pearl. Cleanness, Patience and Sir Gawain. (1923). Early English Text Society, 162. Reproduced in Facsimile from the MS. Cotton Nero A.x with Introduction by Sir I. Gallancz.