After Flannery. He's a normal gym.
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You can fight your father in the Petalburg Gym after you obtain 4 gym badges from other cities. -Wiz
after you beat flannery hes a normal type gym
the town west of mauville
On Pokemon ruby to get surf you need to beat your dads gym (in petalburg) then after wallys dad will come and get you then give you surf.
go to petalburg city,fight with your dad first.then at the right u will find house go to the house then talk to the man hope my answer is correct.