GV wasn't even an internal product - they bought up Grand Central and for a while the business offering was still branded as GC. But at one point, it was integrated into Hangouts. It's so strange...
They don't support any HD voice codecs, so call quality is shit, the integration with iOS is non-existent, the app doesn't sync in the background so every time you open it you have to wait 5-10s before you can use it...you can't even set a ringtone so you can tell a GV call from a cellular call.
There are years between updates and then suddenly for like 6 months it'll get a slew of updates, most of which aren't user-facing. Then back to update silence. It's really odd.
I'm still amazed that google hasn't killed it.
Or Gvoice is some backbone service that something else uses. Idk, but it has to be used for something for Google to have not killed it yet.