Our mobile SDKs include delegates which are basically equivalent to the callbacks on the web for certain events taking place in Composer. These events on mobile are:
showLogin
showTemplate
nonSite
userSegmentTrue
userSegmentFalse
meterActive
meterExpired
experienceExecute
Each SDK is documented and includes these events in the documentation:
Please note there will be some differences between mobile and standard Composer because we aren't handling terms or checkout through Piano for mobile products (those are handled via the app store, through Apple iTunes or Google Play). So, you won't see an equivalent event for termSelected or checkoutSuccess, for example.
For a completed in-app purchase, you'd need to leverage a success response from the app store after purchase or something along those lines, to deliver that data to GA.
For Piano events, however, you can leverage the events that are available in the mobile Composer to send data to Google Analytics (GA).