A sixteen line ABAB rhyme scheme would most likely be constructed as four quatrains of four lines each. Each quatrain would have the 1st and 3rd lines rhyming and the 2nd and 4th lines in a separate rhyme. [A quatrain is four grouped lines, a four line stanza, having any number of possible rhyme schemes.] The next quatrain would start over again with a new ABAB scheme. (You might representatively think of it as a CDCD scheme in the second stanza to clarify that stanzas following the first do not carry on with the same rhyming sounds.)
It is very common for poets to write out their ideas in prose and then to versify the ideas afterward. If you want a sixteen line poem commenting on Gawain and the Green Knight, you might think of expressing your emotional reaction to events or your opinion of Gawain or your opinion of the Green Knight or the suspense and anticipation you felt at the climax.
As an example, you might do something like the following in which I envision myself in the company described in the first stanza of Book I: Christmas in Camelot:
I stand in great awe
King Arthur walks by;
The glories I saw,
None ever did spy.
In this ABAB quatrain, I rhyme awe with saw and by with spy: ABAB.
[It is written in anapestic dimeter with a variation omitting an unstressed beat at the beginning of each verse (line), the unstressed beat being replaced by a pause.]
I^ stand' / in^ great^ awe'
King^ Ar' / -thur^ walks^ by';
The^ glo' / -ries^ I^ saw',
None^ e' /-ver^ did^ spy'.