Gengeng is Northeastern Chinese slang for talking back in a low, grumbling voice, usually because you are not convinced. The speaker does not argue openly but mutters objections as they go. 'What are you grumbling about?' is the usual line that includes it. Small Town Remedy's first trailer caption asks who is doing the gengeng, a nod to the hero's problem with local speech.
Dramas
Where it comes up
Neon & Nine-to-Five
Small Town Remedy's hero can't follow his hometown dialect
Xiao Zhan's doctor in Small Town Remedy comes home to Northeast China and can't follow the local dialect. Viewers are already arguing over whether the show's Northeastern is real, and Cantonese speakers say his Cantonese isn't either. That double miss may be the point.