What is the legal ruling on a group of Muslims gathering money and visiting a Muslim, sitting with him to eat, drink, and remember Allah Almighty, in order to console and comfort him after the mourning period has ended? And is this considered an innovated bid'ah or a good sunnah?

What has been mentioned concerning this act falls under the category of innovation (Bid‘ah) and is not part of the Sunnah. It is desirable to offer condolences to the deceased’s family without gathering for that purpose. There is no harm in assisting them with money or food. However, gathering for condolences and turning it into an occasion for the remembrance of Allah falls under the category of newly invented innovation (Bid‘ah Muhdathah).