Bible Gateway Plus is accessible in the Bible Gateway app for Android, Kindle Fire, and iOS devices. In order to access the Bible Gateway Plus features in the app, you need to sign in. To do so in each app, tap the Menu icon in the upper left corner, tap Settings, and tap Account and Profile. Tap Log In with Bible Gateway and enter in your e-mail address and password. Tap Log In. You'll now be able to access all the features of Bible Gateway Plus that are available in the apps.
For more information about using the Plus features in the app, please refer to this iOS article and this Android article.