Certain things are built into the simulator and might not show up in the wiki – in those cases we try to document them or just answer questions when people ask them.
For the minimum GCD, in general it is 750ms. We also allow that to be overridden on a per-spec basis. You would find that on the spec definition, e.g. https://www.askmrrobot.com/wow/theory/mechanic/class/deathknight?spec=DeathKnightBlood&version=live
That page will also define the general GCD for that class/spec as well (1.5s for most specs, 1.0s for others).
(I don’t know of a spec off the top of my head with a different minimum GCD… Swol might, if there is one.)
Also, particular spells can have an alternative minimum GCD (or cause a different length GCD in general). That would show up on the particular spell in the wiki and will override the above.