Went ahead and tested this.

Method: Took the Gargantuan outside of New Tristram and watched/timed him as he punched zombies. Removed and Reequipped items to test INT, Weapon Damage, and Attack Speed gains and timed their effect on the stun.

Findings:

• Against a single target the Gargantuan stunned the enemy around every 10 seconds. Against a group he appears to do the same.
• The stun appeared to last around 3.5 seconds.
• INT, Weapon Damage, and Attack Speed increases did not appear to increase how often the Gargantuan would stun or the duration of the stun.

Went ahead and tested this.

Method: Took the Gargantuan outside of New Tristram and watched/timed him as he punched zombies. Removed and Reequipped items to test INT, Weapon Damage, and Attack Speed gains and timed their effect on the stun.

Findings:

• Against a single target the Gargantuan stunned the enemy around every 10 seconds. Against a group he appears to do the same.
• The stun appeared to last around 3.5 seconds. Probably due to lag and attack animations.
• INT, Weapon Damage, and Attack Speed increases did not appear to increase how often the Gargantuan would stun or the duration of the stun.