Alliteration is the repetition of consonant sounds at the beginning of words or stressed syllables in a phrase or sentence. Here, the repeated "g" sound in "gown," "grassie," and "green" creates a musical and memorable effect.
The line also uses imagery to evoke a visual picture of a gown with flowing, green sleeves. The use of the word "satten" suggests a luxurious and elegant fabric, further enhancing the image.