Cordial Sino-Japanese relationship is a sham. China cannot possibly reconcile with Japan, and neither would Japan really trust China. The whole diplomatic dance is nothing more than biding time to hit the other after gaining enough strength.
Gyoza incident is just another aggravation. It is not a tide-turning event. Both Chinese and Japanese have enough motivation to perpetuate this incident. Whoever turns out to have done it, both country will vehemently deny its countrymen are responsible.