Here is the full list of all 11 Gunhound EX achievements.
Maxed out the Score Counter
Acquire the Anti-Ship Energy Cannon
Acquire the Grenade Launcher and Hand Grenade
Acquire the Shotgun
Acquire the Clutch Fang
Acquire the Napalm Thrower
Acquire the XVI Naval Cannon
Acquire the Gatling Claw
Cleared all Simulator Missions
Acquire the 30mm Autocannon
Acquire the 35mm AA Autocannon