I'm not able to answer your question, but I think you'll have a better chance of finding someone who can do if you edit your post and tag it with Ensemble and HealthShare. Also worth mentioning that the "Developer Community" tag is primarily intended for feedback about the DC software itself.

Here's the base implementation of this method in %Library.GlobalIdentifier (2017.1 version):

 /// If a persistent class is GUIDENABLED then this method can be called to override the default GUID assignment.
/// This method accepts a guid value and returns the override value currently assigned. The return value will only
/// differ from the supplied value if the override is unsuccessful. It is only valid to call this method on a new object.
/// The guid value passed to this method is not validated. It is up to the user to make sure the guid is properly formed.
/// The guid assignment does not actually occur until the object is saved. If the object has already been assigned a GUID
/// or if the GUID override value has already been assigned to another object then the GUID override value will be discarded.
/// The check for GUID value uniqueness and a GUID value previously assigned is done only at the time the object is saved and is
/// not performed by this method.
Method %OverrideGuidAssignment(pGUID As %Library.CacheString) As %Library.CacheString
{
set i%"%%GUID" = pGUID
quit i%"%%GUID"
}

If I understand Alex correctly, he's calling this method in order to force a new instance of a GUIDENABLED class to have a specific GUID instead of getting one automatically assigned at save. He's not overriding the class in his own class definition.

I have highlighted in yellow some sentences in the comment that may be relevant.

I have also highlighted in pink the sentence Alex quoted, which is a bit unclear. By "return value" I think it means the GUID of the object instance after the save has subsequently been performed.

In case you haven't already reviewed it, this article by @Murray Oldfield contains some debugging tips that could be useful.

Unless you upgrade to 2016.2 or beyond you can't yet use Atelier as your code editor, so integration with file-based source code management (SCM) tools such as Git or Perforce isn't so straightforward. Deltanji works natively inside Caché 2008.1 and later and integrates easily with Studio (as well as with Atelier). The Solo edition is free.

Or maybe when you write "Configuration management tools" you aren't referring to SCM?

In the same way as you create an instance of any class:

ENSDEMO>set oInst=##class(Demo.Loan.FindRateProduction).%New()
 
ENSDEMO>zw oInst
oInst=<OBJECT REFERENCE>[1@Demo.Loan.FindRateProduction]
+----------------- general information ---------------
|      oref value: 1
|      class name: Demo.Loan.FindRateProduction
| reference count: 2
+----------------- attribute values ------------------
|  AlertActionWindow = 60
|AlertNotificationManager = ""
|AlertNotificationOperation = ""
|AlertNotificationRecipients = ""
|    ShutdownTimeout = 120
|      UpdateTimeout = 10
+-----------------------------------------------------
 
ENSDEMO>

On this post the answer by @Dmitry Maslennikov might be useful for you.