I suppose we're going to say this is Hard cover.
According to the RAW (RMFRP pg 214) it's +30DB for Directed Spells for 1/2 to Partial "Hard" cover.
But maybe it should be different when we consider physics.
The Sorcerer's Apprentice (Disney) movie touches on this point a bit. In the movie, if you're going to work with magical induced 'real' lightning, you should know a bit about tesla coils, faraday cages and physics, in general.
This assumes RM magic effects have anything to do with physics.
If the Lightning comes from Light Law, it might ride some laser solenoid tunnel to it's target. This might keep it on track without losing punch. This way, it may not be distracted by the metal gate, except as standard cover.
It'll be interesting to see how other people respond.