Jez describes a casual meeting. He is walking north on the east pavement where block 6 is, so he needs to cross to the west pavement where block 5 is as he's going to block 4, which is behind block 5. In doing that he meets Gerry, who is using that pavement to go south from block 5 to the Tapas.
Gerry's desciption includes an intentional diversion to speak to Jez. He crosses to the east pavement to do that, then crosses back again to go to the Tapas.
All the following is IMO based on the facts as I know them.
If I was Gerry I would be in a different frame of mind. They had that incident with Madeleine the night before where she had a tea stain on her pyjama top, and they had thought someone might be "looking after the kids while they were out". He has just been in the room the door had been moved from where he had left it, "more open" than before. Gerry IMO was fully aware of the possibility of someone hanging around the apartment, so he was in the "fight mode".
He gets out onto the street and sees someone, he approaches and finds it is someone he knows. Was his suspicions wrong? One way or the other he has to calm down and converse since he has approached Jez and Jez is a big opponent if there was to be a fight 6' 3" was the number said.
That is a problem, six foot 3 inches of trouble! No wonder he doesn't see Jane pass by.