Loadout:
Cannon
Sawn Off Shotgun

Squaddie Perks:
Suppression - Suppress an enemy target with cannon fire, they get -30 aim and if they move you get a free reaction shot against them.
Shotgun Tap - Grants 'Single Tap' (Fire Sawed-Off for 1 action) & 'Double Tap' (Rapid-Fire Sawed-Off for 2 actions) perks.
Sawed-Off Reload - Reload the Sawed-Off Shotgun twice per mission.

Controller Tree:
Steady Hands - Grants +10 aim and crit if the gunner didnt move last turn and this turn.
Demolition - Destroy enemy cover with the cannon.
Holo Targeting - Cannon shots mark enemy targets granting allies +15 aim against them.
Bulletswarm - Can take cannon shots for 1 action, allowing shoot and move or shoot twice.
Overdrive - Every few turns this ability can grant the gunner an extra action point.
Saturation Fire - Cannon attack in a cone-shaped area that destroys cover and hurts enemies.

Juggernaut Tree:
Shredder - Cannon attacks shred enemy armor.
Fortitude - +10 Defense.
Shotgun Surgeon - +1 Damage, +15 Aim & +15 Crit with the Sawn Off Shotgun
Hail of Bullets - Cannon shot with 100% chance to hit. Uses 3 ammo.
Untouchable - If a kill is scored on a turn get immunity to the next attack against you.
Gatling Fire - Launch a volley of 4 shots against a single enemy.

Watcher Tree:
Cool Under Presure - Overwatch shots get +10 aim and can critically hit.
Covering Fire - Overwatch shots will trigger on enemy actions, not just movement.
Seek and Destroy - 50% chance to attack any newly revealed enemies that come within range.
Long Reach - Can take cannon normal/overwatch shots against distant enemies in allied sight as long as there is line of sight.
Make It Count - Grants +20 aim on overwatch shots.
Kill Zone - Lay down a zone of cover (needs both actions). Enemies that enter this zone will trigger an overwatch shot from the gunner as long as the gunner has ammo to do so.

GTS:
Resolve - +10 Defense & Willpower.