Not even the whole power of the corrupt Grail, which is way stronger than any CSs, can force Gil to do something he doesn't feel like doing.

And that's not even counting his Magic Resistance, and numerous ways to counter every conceivable magic in existence by using his treasury. Or, the fact that would know it would happen before you even think about it.

The same would apply to trying to go on an arms race against this city. Even if you take only Gil into account, there's no way they could keep a summoning system meant to harm the city out of his knowledge, which just means that they are inviting his wrath. Would the destruction of everything he feels like destroying as a pay back be really worth it?