Most often, Gospel Library updates when the content changes. This would be from changes or corrections to existing content, or new content, such as Church magazines each month or new lesson manuals as they are developed. The Android developers made the decision to push updates every time there is a change, so that GL will be as current as possible. I do not know if iOS does the same, or if they push content on a more infrequent schedule.
The new version of GL also seems to phone home every time you open the app. I believe this is to synchronize annotations and other highlights, and perhaps for other reasons. This is different from a content update, but since it happens so often and the content updates are so frequent, it seems as if GL is always updating.
In the Android version, while an update is taking place you can click on the progress bar and see what is coming your way. You can get more precise information in the topic under Gospel Library for your particular mobile device.