This is the 730th most frequent Turkish word.
gönüllü
"Voluntary" or "volunteer".
Here, 'gönüllü' is used as a noun meaning 'volunteer' to describe the subject's role at the charity.
O, bir hayır kuruluşunda gönüllü olarak çalışıyor.
He works as a volunteer for a charity.
In this sentence, 'gönüllü' is used as an adverb to indicate the voluntary nature of the action.
Bu projede gönüllü olarak yer almak istiyorum.
I want to participate in this project voluntarily.
Here, 'gönüllü' is again used adjectively to describe the manner in which the help was offered, emphasizing willingness.