These versions change frequently, but the currently supported versions of iOS and Android for GTViewer are shown below.
The current version of GTViewer for iOS is 3.0.70.
Supported versions of iOS are 9.0 to Current (which is 11.0.3 as of this post).
Support for iOS 8.0 has been dropped.
The current version of GTViewer for Android is 3.0.23.
Supported versions of Android are 4.4 (KitKat) to Current (which is 8.0.0 Oreo as of this post).
Support for Android 4.2 (Jelly Bean) is available but Web Apps and Data Collection are not supported (because required functionality to support them is not provided until Android 4.4).