I should mention that it's likely that the server also performs it's own GCD checks to prevent people with hacked clients from being able to peel off GCD and dish out faster damage, and with latency being variable you're likely to have things fail to cast if you're too precise when it comes to recasting right as the GCD finishes.

Also, if you cast something that will be rejected by the server your client will still go into GCD mode and prevent you from sending any other casts. When the server rejects the cast, the client will cancel that GCD and allow you to send the next thing. This is why casters with high latency (hello, fellow Aus/NZ players!) are better off carefully choosing when to recast instead of spamming the button - if you have a 400ms ping and you try to recast 100ms early your client is going to go into lockdown and not send anything else for the next 400ms until the server comes back saying "You can't cast that, you're already casting something", which means that your next cast will be delayed by 300ms. That's actually a biggish chunk of a spell (15% of a 2s lightning bolt, for example) to be losing regularly and will probably cost you a couple of slots on the damage meters.