I'm not quite sure why the devs haven't added it in yet as a craftable weapon.
http://www.fo2238.fodev.net/wiki/Gatling_LaserLet me put it this way.
The gatling laser, as it is, is not as good as it seems. Those damage values? They mean nothing when you look at the burst shot size and the DR on armor.
20-40 turns into 8 damage per bullet (80 per point blank burst) against Metal Mk2, a very common armor. Then, there's 7DT on the armor, (6DT at 1% deterioration)
This means, the gatling laser, at point blank, will only do up to 74 damage in one burst before Bonus Ranged Damage. Which only adds 20 damage each time you take the perk, as opposed to 60 on a minigun. Miniguns easily do 300+ damage to Metal/Combat Armor wearing players because of the lower Normal DR on armors and the fact that their ammo has -AC and -DR modifiers.
So why hasn't it been added as craftable? It would make more people take energy weapons and would add more diversity to the gameplay instead of just expecting every single PKer to have either a gatling gun (Or rocket launcher), or a scoped hunting rifle (Or sniper rifle).
Now that that's out of the way, I even have a suggestion on what the crafting requirements could be!
Gatling laser requirements:
Energy Expert [3]
Minigun
Laser Rifle (Ext. Cap.)
3 Electronic Parts
1 Quality Metal Parts
Does that not seem fair for bringing more balanced weapon choices to the game? Do keep in mind, that currently, energy weapons only has 4 weapons to choose from, one of which is useless if you aren't a sniping build (The laser rifle, because of laser damage resistances) and most people would not sacrifice the ability to do aimed shots with a plasma rifle, for the 1AP lower on gatling laser bursts.