how to earn gem really fast?

The title says it all

by contracts is the easiest

Write a buttload of articles and hope some or all get accepted one day. It would be nice if they'd sent a letter of refusal, so that I at least know that they have read one of my articles.

Complete contracts on the Wall Of Terror.
Take a bunch of'em the same time, because most of these are city buillders and you'll have to wait sometimes for a building upgrade or resource build up or some other process to finish, use that time to advance in towards the completion of multiple tasks for multiple contracts.
Don't forget to take screenshots everywhere every minute and always think of a way you can prove that it really is your screenshots when claiming your credit. If you don't know what to do just ask, in the forums, w'll gladly help.
When choosing a character name or any other names , use your email name whenever you can.
After you complete the task, go claim your credits. Go down the Wall Of Terror and click on "Do you have any issues with your task? Click here". You'll be redirected to a page where you can see all the contracts you started , now click on the button next to your completed task, in the new page, provide your Email address and send a report of completion, just write something like "i've completed the contract, I'm waiting for my credits". Go to your Email Box, you'll recieve almost instantaneously an Email asking for proof of completion, now respond with the screenshots you took. If your screenshots are good, you'll receive an Email in less than 24h saying your task has been accepted, and your SGS won't take long after that.