That error shows up because minreal() and ss(), etc. are functions in the Control System Toolbox. Those functions cannot accept sym objects like Gsys.
Even if you just use Control System Toolbox functions, you're going to have troubles because the elements of Gsys are improper (degree of numerator > degree of denominator).
Also, I thought the Smith form only applies to the "numerator" of the transfer function matrix, not the entire tranfser function matrix.
Also, doesn't that call to smithForm throw an error because Gsys isn't square?
What exactly are you you trying to do? Find the Smith-McMillan form of Gsys?