The problem with gargoyles isn't that they fundamentally can't fly; I don't know. It's that mages are, in theory, smart guys and should understand that it's better to have:
1) light objects
2) objects with a larger surface area
3) and generally good aerodynamics
Gargoyles seem to be deliberately created in defiance of all these rules. At the same time, they are a cheap and numerous Tier 2 unit, meaning it's easy for mages to create such suboptimal things. This raises the question: if it's so easy, why not make flying golems and titans, for example? Well, add 10-20 gold to the cost, or even make it free as a bonus.
Why can't we make the same gargoyles, for the same price, but with good aerodynamics, i.e., faster and stronger?
In short, no matter how you look at it, it's some kind of nonsense, and magic doesn't save the situation.