In a glad manner; happily.
"She gladly accepted the invitation to join her friends on their weekend trip."
In plain English: Gladly means doing something with great happiness and willingness because you really want to do it.
"She would gladly help me move if I needed an extra hand."
Gladly comes from Middle English and Old English, where it originally meant "in a happy or cheerful manner." The word traveled into modern English unchanged in meaning, formed by adding the suffix "-ly" to the adjective "glad."