Now the term gravely ill typically means that you have an illness and the prognosis does not look good. You would, unfortunately, be facing a high risk of death. Now that certainly doesn't guarantee death, but the situation is very serious and worrying.

What kind of word is gravely?

Gravely is an adverb. The adverb is an invariable part of the sentence that can change, explain or simplify a verb or another adverb.

What does gravely worried mean?

(formal) ​in a very serious and important way; in a way that gives you a reason to feel worried. She is gravely ill. Local people are gravely concerned.