GCD Fire Mage

Hello, everyone. I gathered here all the smartest and skilful fire mages of the project to explain to me.

1. Do you need speed on a fire mage, if yes, then how much?
2. How much is the minimum gcd for casters and how to achieve it on fire mage?

And the main question

3. How is gcd calculated?

Sorry, I forgot to add.

This is all I'm specifying for pvp fire mage (if it's important in this context).