Professional enough they had a man in a secret service disguise. Classic Mafia.
So, you start off by saying - "Maybe not professional" - and now it's - "Professional enough"
I assume you believe there was a shot from the GK and you're just going to ignore the points being made about how ludicrous an idea that is in terms of planning the assassination.
The GK could have been swarming with people, there could have been hundreds of people in that area. The planners had no way of knowing what they would be dealing with. There could have been hundreds lining Elm Street. And behind the picket fence was a car park - people parking cars, returning to cars etc.
The car park area was wide open. Look at the pic below, there is no way to control this area:

Try to put yourself in the position of someone planning the assassination.
How sensible does a shot from behind the picket fence look now?
One of the main reasons why people believe there was a shot from behind the picket fence is because so many people run up to that area immediately following the shooting.
Motorcycle cop, Clyde Haygood, heard this transmission on his radio from police Chief Curry:
"Get a man on top of that triple underpass and see what happened up there".
That's exactly what Haygood does. He gets off his bike and races up to the triple underpass - not up behind the picket fence.
A group of people see a cop running up there and decided to follow and this causes a cascade effect of people running up to that general
area just because they've seen other people running up to that area.