Havent personally seen one on a GMC mower, I do belive though the quantum was still usa made up until recently, but after the early 2000-01 era quality of the materials droped off severely.
As to which country builds the worst engine, I purposely ran a chinese one very hard with normal servicing intervals and it has around 370 hours on it and is getting very sad, almost to the point where I will scrap it soon. All of my usa made oldies, some are begginning to smoke but the good ones I have are up to 40 years old and have barely any wear inside them.