I have a high powered LED light that I'm wondering if it would affect the GPS or any of the electronics if I attached it via magnet to the bottom of the Spark.  

Hmmm...magnet sounds like a bad idea.  Also, don't come close to covering the bottom sensors, or your Spark may launch up and try to outrun it...crashing into something, or never being heard from again...(I've experienced this).

I wouldn't attach anything - especially a magnet.  It will affect the compass and calibration. Additionally, it will affect aerodynamics and you'll risk having your drone fly off and out of control.

Magnets and compasses don't go well together

That definitely sounds like a bad idea