If I play Stupefy to an enemy unit at the Battlefield Void Gate, does it deal minus 2 or minus 1 might?

Playing Stupefy on a unit at the Void Gate will reduce that unit's Might by 1.

The interaction does not result in -2 Might because Void Gate specifically increases damage, whereas Stupefy is a Might-reduction effect.

According to the card text:

• Stupefy: "Give a unit -1 might this turn, to a minimum of 1 might."
• Void Gate: "Spells and abilities deal 1 Bonus Damage to units here."

As noted in the official FAQ regarding Imperial Decree (which prevents damage): "Stupefy does not kill units... because Stupefy does not deal damage." Since Stupefy does not deal damage, it does not trigger the "Bonus Damage" effect of the Void Gate. The unit will simply lose 1 Might (to a minimum of 1).