Both jīd and ʿunuq can mean neck, but jīd is especially common in elevated descriptions of beauty and proportion. The particle ka- in ka-jīd al-riʾm compares the woman’s neck with that of a white gazelle.
Subject: the woman’s neck. Comparator: the gazelle’s neck. Shared qualities: grace, smoothness, proportion, and upright poise. The comparison does not transfer the animal’s voice, speed, or wildness.
The preceding verse borrowed an animal’s gaze; this one borrows its neckline. Distinct features from the natural world thus serve successive parts of the portrait.