Is ghusl obligatory for a girl who saw or imagined intercourse in a dream and felt pleasure, whereupon a white fluid was discharged, and she felt languid after its discharge?

The thoughts that come to you involuntarily are not sinful. However, inviting them and indulging in them is not appropriate. The discharge that typically occurs during thinking is usually madhiy, which necessitates wudu' and not ghusl (definition). Ghusl is only obligatory if there is certainty about the emission of maniy. If you doubt whether the discharge is maniy or madhiy, you may choose and assign it the ruling you prefer. This is the view of the Shafi'i school.