In a gloomy manner; in a way that is dark, sad, or melancholy.
From 'gloomy' plus '-ly' (the standard adverbial suffix), converting the adjective into an adverb describing how an action is performed or a state exists.
The '-ly' suffix is one of English's most productive endings—it can transform almost any adjective into an adverb, which is why we can say someone 'gloomily stared' or 'gloomily predicted.'
Complete word intelligence in one call. Free tier — 50 lookups/day.