This is the 5573rd most frequent Japanese word.
ぎゅ
A hug or squeeze.
The word "ぎゅ" is used here as "ぎゅっと" to convey the meaning of tightly or firmly, indicating strong grasp or pressure.
彼女はぎゅっと手を握った。
She tightly held my hand.
In this sentence, "ぎゅ" emphasizes the manner of hugging as firm and affectionate, expressed as "ぎゅっと".
ぬいぐるみをぎゅっと抱きしめた。
She hugged the stuffed animal tightly.
Here, "ぎゅ" describes the sensation of sharp pain, again used in "ぎゅっと" to indicate its abrupt simplicity.