What makes a question such as this one tricky is that, with rare exceptions, most readers of Sir Gawain and the Green Knight will not be working with the original document, which was written in Middle English, but rather with a translation. However, there are numerous translations available, not all of which have preserved the poem's original bob-and-wheel meter (indeed, some have even switched to a full-prose translation).

The bob-and-wheel is a short rhyming meter that closes a larger unrhyming metrical passage. To quote the Oxford Dictionary of Literary Terms:

It consists of one short line (the bob) with a single stress, followed by four three-stress lines (the wheel) of which the second and fourth lines rhyme with the bob.

To address this question, I'm using J. R. R. Tolkien's translation of the poem, which is written largely in accordance with that original metrical style. In Tolkien, one example that jumps out as a slant rhyme can be found at the end of the eighth stanza, in which you find the rhymes (and near-rhymes) mode, aghast, showed, passed, glowed. Here, the slant rhymes can be identified in the words aghast and passed (that is, assuming you pronounce the word passed with the d-sound, as it is spelled). In contrast to Tolkien's slant rhyme, changing the word passed to past would produce a perfect rhyme.