[quote]Because nobody is going to get a good nights sleep if they knew a kid with three command seals could wake up tomorrow with a world cracker and have a bad day that results in the entire world going boom[/quote]

Fairly certain the chance of anyone command sealing Gil into trying to destroy the world is nil, they'd be dead before they could finish the command. He's the only world-ending effect, and [i]presumably[/i] being on an island in the Pacific stops anyone from having Karna nuke a country. 

People might be terrified at first but it'd quickly decay to your same-old existential dread as nobody is actually threatening anyone else and this also means some of history's greatest inventors and artists are also around again.

Your biggest insurance against all these things is, ironically, Gilgamesh + Sha Naqba Imuru. Unless he goes obviously insane, that clairvoyance (and anything else like it--there's enough historical prophets) is good advance warning. It's self-policing.

[quote]Would cause such a massive war of culture.[/quote]

Summoning process solves that.