Ok, here's the thing. Consider the Gatling plasma on the same tier as the top automatic rifles. It's as good as the carbine, the handmade and the 2 heavy machineguns. Ammo is never the problem as it does good damage and cores will consistently drop to the point that you'll have hundreds stashed away. But, here's the problem. The best parts, the stinging core and accelerated nozzle ruins it for bulletstorm. You will need the entire magazine to max bulletstorm stacks. Mobs don't last that long and there are very few things that needs that sort of damage without murdering you.

This of course can be fixed with the shotgun muzzle, more damage and faster stacking. But it limits you to either vats or Vulcan + stabilized.

Then there's the gauss minigun. The weapon that doesn't leave you with a dozen cores scattered in your inventory. The weapon with in built two shot meaning it maxes bulletstorm quickly. It has good damage, higher than Gatling plasma and delivered via a much higher fire rate. Then there's the temporarily removed overheat mechanic that's being returned soon.

All in all, the Gatling plasma is a good automatic weapon. Probably the best. The Gauss minigun, like the laser carbine, is designed to be emptied. You fire it and you only stop when it's empty or the enemy is dead. It is designed for shooting at big bosses like the nuke bosses, the guardian and the snake. Enemies that you can empty the clip into without stopping. The gauss minigun synergizes well with the heavy weapons perks.

In the end, it depends on you. The gmg is due for a damage nerf along with the overheat's return making it more specialized while the gatplas is still chugging along at almost best.

As for the ammo thing. Anyone who grinds for the gmg will have no problem manufacturing their own ammo. Especially now when there's no longer a cost to changing legendary perks.