There's a different selection on the tablets than on the Alaska Beyond portal, in that sometimes the tablets will have 3-4 movies that aren't available on the portal, but the portal will in total have many more than the tablets.

Also note that the app you need is not the Alaska app, it's the Gogo Entertainment app which you can find in your app store now. On Android, you can download it from the web portal while in flight. However iOS annoyingly won't let you download an app from a website.