Do not cite the Deep Gigagon to me, Refitter. I was there when it was written.
The reason why people dislike current Gigacannon is it has so little 'right' going for it. It has the performance of a budget large weapon (low dps, efficient) with the OP cost of an expensive large weapon (tachyon lance, plasma), in a weapon class that is dominated by very powerful weapons (autopulser, plasma, HIL, Tachyon). It's also very clunky to use, the turn rate on a turret is abysmal since its 'firing turnrate' activates during its long charge-up, leading to many missed shots if it's not a 1v1 scenario. Hardpoint gigacannon has it even worse. It's not even a good choice on the Ziggurat compared to your other options, since the DPS is so low and your flux budget on the Zig is so high. It doesn't mean GigaZig is a bad ship, just that GigaZig is not as powerful as TachyZig or PlasmaZig or even AutoZig.
I used to be a huge proponent of gigacannon Paragon when gigacannon was first released, but not as a 4 gigacannon paragon but instead as a 2 gigacannon 2 plasma paragon. The overall idea is that gigacannon is very flux-cheap and flux efficient with a long refire time to combo with fortress shield, so that should mean it, on paper, basically replaces autopulser for the 2x auto 2x plasma Paragon.
Unfortunately, testing showed a gigantic flaw for AI use that has made gigacannon basically be a huge waste, particularly the hardpoints but this likely fits for the turrets too, though probably to a lesser extreme. As far as I've observed the AI driving the Paragon does not care if it will hit an enemy when it shoots its gigacannons, only that an enemy is currently in the path of the gigacannon shot when it clicks fire. The charge-up is so long that this means it's extremely likely to just miss. In the hardpoint's case, constantly missing if there's more than one enemy nearby. Which is probably very likely if you're using a Paragon.
Player piloting can avoid this issue and if you do that, Gigacannon is a perfect 'support' weapon for better large energies on ships that have more than 1 converging large energy weapon.
I might pick up the Gigagon again after the update, but this time use it on the turrets instead of the hardpoints. I was hesitant to do that because of the low DPS, but with the increased DPS next patch maybe it'll be a good option in the turrets. It should miss less often there compared to the hardpoints.