its maybe a trojan.. never download links from ppl who dont send virustotal link or are not trusted like this guy(registration april 2014, 3posts)
b2t: atm no gem hack is working. this is because sp(social point), the publisher of dragon city, has improved their firewalls a lot! 'cause of this, the old gem hacks dont work any more :/
keep calm and wait a few weeks... maybe then a new gem hack is released here :)