While, we are not sure about a specific RTM date, it has started seeding to OEMs during last week. We have seen couple of GDR1 versions appearing online and the update may be released as developers preview in August to participating devices.
Some devices running Windows Phone 8.1 GDR1 natively may be due for launch in late August to September time-frame and these may include upcoming Lumia devices too.
Sometime back we reported that the ability to create start screen folders natively is coming to Windows Phone 8.1 soon. The new feature which appeared briefly on Microsoft’s Windows Phone 8.1 support page was captured by us before it was pulled and we can confirm GDR1 will have this feature and may have support for “Voice over LTE / 4G”, the latest in mobile voice communication.