When you compare what WeakAura's GlobalCoolDown trigger to the GCD builtin variable in retail, the numbers do not match. Not necessarily saying GSE is wrong, but it probably warrants an investigation.

This WeakAura shows the calculation WA is using:
https://wago.io/r1xLbw4vm
i.e. the formula should be (in seconds):
1.5/(1+0.01*UnitSpellHaste("player"))

For example, my current haste rating is 712, which yields a 21.58% haste.

The GSE is calculating:
math.floor(gcd- (750 * haste / 100) + 0.5) / 1000) which is
math.floor(1500- (750 * 21.58/ 100) + 0.5) / 1000) which is 1.33865

vs:
1.5/(1+0.01*21.58) = 1.2337555519

The formula that I got from Blizzard is that 100% haste lowers the GCD by 750MS from a base of 1500

1500 - (750 * (haste/100))

Can’t remember what the 0.5 was for at the end

Well, when I trigger the cooldown and invoke GetSpellCooldown(61304):
/cast Mend Pet
/run local _,d = GetSpellCooldown(61304); print(d)

And I get 1.233, which is the same the duration in the formula above

See https://wow.gamepedia.com/API_GetSpellCooldown

Ive updated the GCD formula to this one.