Is it permissible to perform ghusl for seminal emission if it was merely a wet dream without physical contact?

No. Nocturnal emission (احتلام) differs from masturbation (عادة سرية) in ruling and meaning. Nocturnal emission is when a sleeping person dreams as if they are engaging in intercourse, and if they wake up and see semen, then ghusl (ritual bath) is obligatory. There is no sin in it, and it does not invalidate the fast.

As for masturbation, it is self-stimulation while awake, and it is forbidden. Ghusl becomes obligatory if semen is discharged, and it invalidates the fast if it occurs during the day and semen is discharged.