It was merged into the pgtk branch by the author of that branch. I’m on that
branch, and when I C-h f framep, I can see the addition to the docstring.

Stefan Kangas <stefankangas@gmail.com> writes:

> “Colin Woodbury” <colin@fosskers.ca> writes:
>
>> Hi there, this patch targets the feature/pgtk branch. Please let me
>> know if the patch is in the correct format, otherwise I’ll resubmit.
>>
>> Thanks!
>> Colin
>> diff –git a/src/frame.c b/src/frame.c
>> index eb5aed82f7..0e74bb937f 100644
>> — a/src/frame.c
>> +++ b/src/frame.c
>> @@ -225,7 +225,8 @@ Value is:
>>   `x’ for an Emacs frame that is really an X window,
>>   `w32’ for an Emacs frame that is a window on MS-Windows display,
>>   `ns’ for an Emacs frame on a GNUstep or Macintosh Cocoa display,
>> - `pc’ for a direct-write MS-DOS frame.
>> + `pc’ for a direct-write MS-DOS frame,
>> + `pgtk’ for an Emacs frame running entirely in GTK.
>>  See also `frame-live-p’.  */)
>>    (Lisp_Object object)
>>  {
>
> AFAICT, this patch was never followed up on emacs-devel?
>
> Was it installed?