No people sit in a gathering in which they do not remember Allah and do not invoke blessings upon their Prophet, except that it will be a source of regret for them. If He wills, He will punish them, and if He wills, He will forgive them

Encouraging Dhikr (remembrance of Allah) and highlighting its merit.
The merit of gatherings in which Allah Almighty is remembered and His Messenger (may Allah's peace and blessings be upon him) is mentioned, and that gatherings without these are ominous for their people on the Day of Judgment.
The warning mentioned against heedlessness of remembering Allah is not restricted to gatherings; rather, it includes other things as well. An-Nawawi said: It is disliked for one who sits in a place to leave it without remembering Allah Almighty therein.
The grief they experience on the Day of Judgment is either because of missing the reward and recompense for not utilizing time in obeying Allah, or because of incurring sin and being subject to punishment for spending the time in disobeying Allah.
If this warning is against heedlessness caused by what is permissible, then what about the prohibited gatherings that involve backbiting, talebearing, and other prohibitions?!