Some translations consider the word μυστήριον (mustērion, “mystery”) a part of the name written (“Mystery Babylon the Great,” so KJV, NIV), but the gender of both ὄνομα (onoma, “name”) and μυστήριονare neuter, while the gender of “Babylon” is feminine. This strongly suggests that μυστήριον should be understood as an appositive to ὄνομα (“a name, i.e., a mystery”).
The verse reads:
καὶ ἐπὶ τὸ μέτωπον αὐτῆς ὄνομα γεγραμμένον, μυστήριον, ΒΑΒΥΛΩΝ Η ΜΕΓΑΛΗ, Η ΜΗΤΗΡ ΤΩΝ ΠΟΡΝΩΝ ΚΑΙ ΤΩΝ ΒΔΕΛΥΓΜΑΤΩΝ ΤΗΣ ΓΗΣ. (W&H)
I am not sure I understand the argument. It talks about the gender of μυστήριον as neuter, but it’s a noun not an adjective. It’s not like John could have changed the gender. Also if this was apposition, wouldn’t it be more likely that he would have written:
ὄνομα μυστήριον γεγραμμένον ΒΑΒΥΛΩΝ …