Hi @Bombarie! This might have something to do with support for ActiveSync with Gmail. Google used to support ActiveSync for iOS and I believed grandfathered in previous customers using this...but it's not supported for new customers moving forward.
If you're looking to sync more than just email, I would suggest trying to set up a Managed App Settings for the Gmail app on iOS. This should enable the Gmail account on the device AND allow the user to go in and turn on sync for contacts, calendars, etc.