Glucagon  is a specific antidote for beta blocker  toxicity:

• in  beta-blocker poisoning  beta receptors are blocked in heart leading to decrease in HR, FOC and conduction
• so GPCR of Beta 1 receptor in heart is blocked, so what to do now?
• give a drug which acts through different type of GPCR
• So,  Glucagon acts through its own GPCR and independent of the β adrenergic receptor and has chronotropic and inotropic effects on the heart
•  in beta-blocker poisoning  glucagon is a physiological antidote

note:

why not atropine an antidote as it also increases HR and conduction

• it is not much effective compared to Glucagon, and doesnot increase FOC 
• it is used if there is bradycardia