Technocrabs
They latch onto nearby cars, effectively transforming them into metallic zombies- explosives required to kill unless you are able to get behind it and burn the headcrab off, as it is immune to bullets but not fire.
However, IF you choose to blow it up from in front, a grenade...
But what about the gun itself?
The gun looks rather similar to the Gravity Gun in HL2, and in HL2E1 Alyx even says the Dark Matter Gravity Gun "looks like it could shoot portals."