I really doubt that. That way of thinking about a GM is a bit dated.
To give an example. Previously software often needed to be put into some physical form and delivered in that form together with the product. It could be a CD-ROM, or an USB stick. That process takes time, and thats why the GM needed to be ready some time in advance. This is no longer the case. The software comes preinstalled and upon first launch it can be updated to the latest build over the internet. Most of the times that actually happens with iPhones. They are shipped with iOS XX.00 but when delivered there is already an XX.00.1 update ready for the device. I remember this was the case for my iPhone 6 back then.
Secondly. From when Apple presents the device there is often 10-14 days until the devices can be bought. They are of course already in production, but they will not be loaded with an iOS before the time around the presentation and then be in transit the last 10-14 days before they go on sale. And like I said in the beginning, the iOS 11 version they'll get shipped with, might not have as high/new a build number as what defined as the final GM build, because it doesn't really matter. It will just be updated on first boot over the internet.
If we again take the iPhone 6 and iOS 9 as an example - The first iOS 9 beta came at July 9th 2015 (WWDC) and the presentation of iPhone 6 was on September 9th. That is just around 90 days, but the GM build of iOS 9 was first released 100 days after WWDC
So there is plenty of time for more bug fixing and beta builds. Don't worry