Gating a post is currently restricted to language and country only. If you use gating on a message, the message won't show up in the feeds of a user outside of the gating criteria nor will it be visible for them on your own page. For example, Belgium is a bilingual country. You want the French-speaking people to see the French posts and the Flemish-Dutch-speaking people to see the Dutch posts. Even when they go to your page. By using gating you'll make sure they see the right content.
Targeting, on the other hand, makes sure the post will only be visible to those in the targeting segment of the page. However, if a fan shares your post, the user's friends will be able to see the post even though they're not in the targeting segment. Plus, the post will also be posted to your own wall, so fans are able to see the posts when they visit your page.