IPA: /ˈɡruːsəmli/
KK: /ˈgruːsəmli/
In a way that is very unpleasant, shocking, or causing horror.
The movie was gruesomely realistic, making many viewers feel uncomfortable.
Gruesomely → It is formed from "gruesome" (from Middle English *gruesom*, meaning causing horror or disgust) and the suffix "-ly" (meaning in a manner of). The word "gruesomely" means in a manner that causes horror or disgust.
Think of something that causes horror or disgust ('gruesome') and remember that 'gruesomely' means doing so in that manner.