Editions

Sir Gawain and the Green Knight. ed. Tolkien and Gordon. 3rd ed. (rev. N. Davis).
The Poems of the Pearl Manuscript ed. M. Andrew and R. Waldron.

You may also find Barron's parallel text and translation helpful.

Some Critical Material

L. Benson: Art and Tradition in Sir Gawain and the Green Knight (1965)
J. A. Burrow: A Reading of Sir Gawain and the Green Knight (1965)
D. Aers: Ch. 4: 'In Arthurus days' - Community, virtue and individual identity in Sir Gawain and the Green Knight in Community, Gender and Individual Identity (1988)
Geraldine Heng, ëFeminine knots and the other: Sir Gawain and the Green Knightí, PMLA 106, 500-14.
*A.C. Spearing in Ch. 7 of Essays on Medieval Literature ed. J. Burrow (1984). Also the Gawain Chapter in The Gawain-Poet: A Critical Study (1970).
Ann W. Astle: 'Sir Gawain and the Green Knight: a study in the rhetoric of romance' JEGP 84 (1985), 188-202.
S. Trigg, ëThe Romance of Exchange: Sir Gawain and the Green Knightí Viator 22 (1991): 251-66.
*D. Brewer and J. Gibson, ed. Companion to Gawain-Poet(1997)
articles by Jill Mann, Sheila Fisher in Medieval Literature ed. Stephanie Trigg. Longman Critical Reader (1993)
Ad Putter, An Introduction to the Gawain-Poet, Longman (1996)

some Gawain links:

Gawain resource page
hear opening stanzas
The Gawain room