غِرٌّ describes an open-hearted person who is not habitually suspicious or busy devising tricks against others. It does not command someone to ignore obvious danger or surrender every responsibility.
The word has a range of lexical meanings, including inexperience; in this narration the accompanying word كَرِيمٌ shows that the praiseworthy quality is a generous, unscheming disposition.
The sound distinction is between trusting without malice and trusting without reasonable care. A person can avoid deceit and still protect an entrusted property.