ps command: list processes by group id

Here is what I get when I list all processes with -g option:

ps -g -o pid,pgid,ppid
   PID   PGID   PPID
112140 112140 112139
127467 127467 127466
130547 130547 112140
130551 130547 130547
130557 130557 127467

…

ps -g 130547

displays nothing, while I would expect 130547 and 130551. Same for ‘ps -s ‘.
Why? Is there a way to display processes belonging to the same group (tree)?