What is the difference between get_post_permalink and get_permalink?

The get_post_permalink() funciton fetches the link to a post depending on its “permanent” link plus your custom rewrite rules that changes ?p=123 into for e.g. my-beautiful-sunday-diary. The get_permalink() function is more “basic” but as well more versatile in what it does: For a post_type of

• page, it uses get_page_link()
• attachment, it uses get_attachment_link()
• post, it uses get_post_link()

It also handles the display of terms like category and date permalinks. At the end, it either replaces the “pretty” link in your home_url() or just returns the raw link if no custom rewrite rules were assigned. Finally it attaches a generic filter:

/**
 * Filters the permalink for a post.
 *
 * Only applies to posts with post_type of 'post'.
 *
 * @since 1.5.0
 *
 * @param string  $permalink The post's permalink.
 * @param WP_Post $post      The post in question.
 * @param bool    $leavename Whether to keep the post name.
 */
return apply_filters( 'post_link', $permalink, $post, $leavename );

Hope that clarifies the topic.

ProTip: If you need to change peramlinks in a plugin, go with the specific filters inside get_attachment_link(), get_post_link(), etc. Only if you are either working on a single site and are not planning to distribute your code or if you are writing a plugin targetting only rewrite stuff, then go with the generic filter above. Else you will nuke every theme authors efforts and start a callback priority race.