Without any haste casting time of scorch is 1.5 sec. That's also the length of your global cooldown with 0 haste. Is it correct that your GCD will always last as long as your scorch? As long as it's 1 sec or higher. Basically i'm asking if GCD scales with haste like any other spell.
Yes I think it works this way. To my mind GCD is considered as a spell with a basic casting time of 1.5 sec, except the fact he cannot be shorter then 1 sec (as you say). One thing is sure as long as your scorch incantation will be greather then 1 sec, your gcd will never be longer then the scorch incantation.
As far as I know some classes can't reduce the GCD. Warr for example.
It's more the spell classification rather than the class. For instance, all Hunter shots have the fixed 1.5 sec GCD typical of most physical moves, but their traps and Mend Pet gcd time is reduced by spell haste effects. Exorcism's and Consecration's GCDs for a Ret are affected by spell haste but Divine Storm and Crusader Strike aren't, etc.