In a gushy manner; with excessive sentiment, enthusiasm, or effusive expression.
From gushy (adjective) plus adverb suffix '-ly' (in the manner of), creating the adverbial form.
The suffix '-ly' transforms any adjective into an adverb, which is why you can describe almost any action—someone can dance 'awkwardly' or speak 'gushily'—making English's adverb system incredibly flexible.
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