For me, gehören + Partizip Perfekt is not a 'Redewendung' but a grammatical structure. Gehören + Partizip 2 means must / should be + past participle. In German, it is a "Passiv-Ersatzform" (substitute).
Das Blasphemiegesetz gehört nicht nur abgemildert, sondern abgeschafft. = The blasphemy law should not just be toned down, but abolished.
Rauchen in der Öffentlichkeit gehört verboten! = Smoking in public places should be banned.
Dieses Gesetz gehört überarbeitet. = The law should be revised.
Der Mann gehört eingesperrt. = That man needs locking up.
You cannot transform any German must-passive into gehören + Partizip 2; theoretically, you can do that, but in most cases it would sound strange. As you may have noticed, the gehören-Passiv is used to express the speaker's non-negotiable (ultimate) conviction of what should be done. You will find it quite often in legal contexts.
I give a link to further examples from DWDS.
Members of the educated classes are likely to reject the construction as no good German. It is in fact colloquial.
As only a few past participles occur within a gehören-Passiv, it may seem to be a 'Redewendung', which is, I think, neither correct nor false. One may regard 'erschossen gehören' as an idiom, but one should be aware of the fact that there is more than 'erschossen' that can be combined with gehören.