woocommerce/plugins/woocommerce-blocks/assets/js/blocks/product-gallery/style.scss

.wp-block-woocommerce-product-gallery {
	dialog {
		position: fixed;
		width: 90vw;
		height: 90vh;
		top: 0;
		margin: $gap-largest;
		z-index: 9999;
	}
}