The Proton Grenade is one of the newest Gadgets that was added in during the Ghostbusters (Event) and can only be earned either by completing tiers in the event, or by purchasing them separately for $2.99 
(for 25) or through the Ghostbusters Weapons Pack for $39.99 (for 25).
In short, it's a combination of both the Energy Grenade and the Shockwave Grenade, when first throw, it will become an explosive and if stick onto a target, it will instantly detonate. Meanwhile, if landed on the ground, after 3 seconds, it will detonate (the visual animations will show 2 circles just like the Shockwave Grenade's, except it's in a pink-ish color) and temporarily stun any opponents near the blast radius (just like the Energy Grenade do!).
Trivia
- An inactive one can be seen on a table in the map Bewitched Headquarters!