Tell me please, what does 'Mark five' in the title of that story means? 'Gospel by Mark part V' or 'fifth model of robot'?

It means "fifth model of robot". It's quite a common way (in English) of indicating that an invention or product has been improved. "GBS - Mark One" would mean the very first version of GBS. If Bradbury were writing the story today (and if he knew anything about computers!) he might have called it "GBS v5.0"

Thank you, Phil. In a rather old translation into russian we had it as "model" of robot. But recently the collection was published, where translator named the story as "GBS -- gospel by Mark, part five", thinking that it is allusion to christian Holy Write. Mistake?

Sounds as if things get lost (or added, or garbled!) in translation.