Is ghusl required after seeing in a dream that something has been discharged, knowing that there is usual wetness upon waking?

If you are not certain about the emission of semen necessitating ghusl, then ghusl is not obligatory for you. Similarly, istinja' is not obligatory for you if you are not certain about the emission of urine. Normal discharges are pure but nullify wudu'. If you doubt whether what has exited from you is semen or something else, you may choose and assign it the ruling of whatever you wish.