Alpha Counters (which they're best known as) are moves you can perform while in guardpause. This requires meter to activate and can be used to escape your opponent's offensive pressure, giving you invincibility in the process. The amount of damage it will take can varies by game and character. Of course they're not called Alpha Counters in every game; in The King of Fighters it's call Emergency Blowback/CD Attack, and in Guilty Gear, Dead Angle. And while they're technically separate mechanics, the idea is basically the same.
For that reason, Guard Cancel Attack would be a more broad term.
While a convenient defensive tool, these attacks can be baited and punished by your opponent if they anticipate that one is coming. A similar mechanic exists, known as Combo Breakers, but is used while in hitstun instead.
To code an GCA, add a move in the CMD that can only be done in state 150 or state 152. These are the states for guardstun before pushback. However, a variable must be included for the command to activate at the end of the pausetime. As well as an HitShakeOver trigger. This allows for the attack to be triggered at the correct time, and not awkwardly happening at any point in guardpause. Be sure to reset the variable when the ChangeState occurs as well.
;Light Kung Fu Knee
[State -1, GCA Condition]
type = VarSet
triggerall = roundstate < 3
triggerall = statetype != A ;cannot be in air
triggerall = power >= 1000 ;must have at least 1000 power
triggerall = command = "FF_a"
trigger1 = StateNo = 150
trigger2 = StateNo = 152
var(10) = 1 ;activate when GCA requirements are true
[State -1, Reset Var10]
type = VarSet
trigger1 = stateno = 1050 ;resets variable as soon as GCA starts
var(10) = 0
[State -1, Gurad Cancel Attack]
type = ChangeState
value = 1050
trigger1 = var(10)
trigger1 = HitShakeOver ;activates when guardpause ends