Use the GD > Post Content block or shortcode with the listing’s ID. It shows that listing’s description (the post content) on any page, not only on the listing itself.

[gd_post_content id="123"]

Replace 123 with the listing ID. You can find it in the address bar when editing the listing (post=123). To shorten it, add a word limit, for example limit="40", which also strips HTML.

Show any other field

GD > Post Meta takes the same id option and works with any field key, including custom fields. Add show="value" to drop the icon and label.

[gd_post_meta id="123" key="phone" show="value"]
[gd_post_meta id="123" key="post_content" show="value"]

See GD > Post Content and GD > Post Meta for all options.

In PHP

The description is the normal WordPress post content, and GeoDirectory fields can be read with geodir_get_post_meta():

$post_id = 123;

// Description, with the usual WordPress formatting.
echo apply_filters( 'the_content', get_post_field( 'post_content', $post_id ) );

// Any GeoDirectory field, by its key.
echo esc_html( geodir_get_post_meta( $post_id, 'phone', true ) );