If I perform ghusl while semen is still dripping from my penis, and then I pray, is there any error in that?

If semen is discharged after performing ghusl, it is either a residue from the previous state of janabah, which only necessitates istinja' (washing the private parts) and wudu' (ablution). Or, it could be new semen, which then necessitates ghusl, and prayer is not permissible before it.