The problem is that your main counter is T2 PD (and aeon Guncoms if you're not Aeon), which is likely what the enemy has if they didn't go guncom.
This is absolutely true, but what I'm arguing is that this typically takes more resources than the guncom, and at best leaves both sides at a standstill or at worst lets the Guncom go attack a teammate.
T2 upgrade also provides far more general utility - e.g. if you're near the frontline you can build radar without needing an engineer; if the enemy tries attacking with bombers you can quickly get T1 AA. You can reclaim faster (I've yet to test it properly in sandbox but if you micro it quickly I reckon you can get DPS signfiicantly better than your base gun reclaiming enemy T1 units); in some niche cases you can even capture the enemy T1 PD. You get more health and regen, and if you decide to retreat your ACU from the frontline you can use it as a useful builder (while getting any future upgrades faster).
You also mention the cost of T2 PD + T2 upgrade for defending guncom, but this neglects a number of points:
1 - the utility provided by T2 upgrade more generally (see above)
2 - the cost of all the extra PGens the guncom needed to get before they could start their upgrade. Mass for these is needed much sooner than the mass for the T2 PD, hence costs more (since your mass income increases at a significant rate early game) - i.e. what % of your income * time does T2 upgrade and T2 PD require cumulatively vs more T1 PGens and Gun upgrade? It's not a case of comparing absolute mass costs since you're getting them at different times. The answer also varies depending on the map.
3 - T2 PD will usually be got if getting a T2 upgrade even if the enemy isnt going guncom assuming the map has some sort of choke point/high value location, since it also helps defend against T1 arti (that can outrange T1 PD).
The extra utility will take quite a while to create an advantage if you have to immediately put it to work on defense, by the time enough PD is up, the player with Gun may almost be done on a T2 HQ for tech, something both players are probably going for anyways. Being able to get up radar is nice, but by that point you'll usually have several engineers there.
Those Pgens still work to scale the economy after the upgrade is done, it isn't as if they stop benefitting after theit initial role. Besides which one of my main points is that the energy curve can be softened by frontloading e-storage while the ACU walks to the front. And in regards to point 2, The economy really doesn't scale at all if you're upgrading and then immediately placing down PD. It's not like you'll upgrade a mex when you need to quickly build defenses against a guncom. For point 3, even if you are going to get PD later that doesn't eliminate it's cost. It would be as if I said the same thing but for the Pgens.
You also mention it's in a good spot 1v1 but not team games, but your changes would hurt it in 1v1 more than teamgames - when I go guncom in 1v1 I find it really hard to make it work, as I've got fewer tanks (due to more PGens) and am really vulnerable to a T1 arti attack (assuming I'm upgrading away from my base - if my ACU has to sit in my base I've probably just lost the game anyway on smaller maps). Making it take much longer to upgrade drastically increases the disadvantages on 1v1, and (given your stated aim) actually has slightly less impact on a teamgame (since teamgames are usually on larger maps, hence you could build fewer PGens since the upgrade is spread over a longer time but not have to worry as much about T1 Arti forcing you to cancel the upgrade compared with 1v1).
Actually you typically have way more space and resources per player on ladder on most maps, with the exception being 20x20 ladder where gun isn't used often anyways, and some 5x5 where one player getting it can be a game-ender. Gun and other ACU upgrades are something you typically get later, as units can have much more of an impact. And the ACU needs to be more generalist.
I'm not denying Gun won't be slightly worse in Ladder with the changes (which by the way can be decreased if necessary), and that it won't come slightly later, but for 400 more mass it is still one of the most mass efficient things you can build, and absolutely takes at least as much of an investment to stop.
No, a gun ACU dies to like 15 t2 tanks no problem if it overextends. Or at least gets into such low hp it is basically incapacitated if it has somewhere to retreat to unless it gets an hp upgrade or farms several vets. Even more so if you include shields to waste initial OCs since people arent (or shouldn't be) running around with +1000 overflow at min 10-11.
My point isn't that a Gun ACU is immune to everything, not the least of which 4 times the investment in an HQ and 15 T2 tanks. In my second post I was arguing (admittedly not worded very well) that a unit advantage can overcome a veterancy/hp disadvantage. However in a fight between gun ACUs, this advantage is reduced significantly unless you have nearly twice as many tanks.
Gun upgrade rush min 5 is only going to be countered by gun upgrade or t2 upgrade yeah, nature of clustered teamgames because you have nowhere to emphasize a unit advantage and so generally stacking utility upgrades on the ACU is the best move to gain or hold ground. Gun is more expensive than T2 and should generally require 5 more pgens, this translates to about 70% of the cost of a t2 pd alongside a later upgrade finish.
Even if the extra pgens needed was 100% of the cost of a PD, I would consider it advantageous, as once the upgrade finishes, the energy is going towards something else.
I'd also say clustered teamgames already are disgustingly static but that's mostly because of how the t3 stage plays and there being essentially no counterplay against a ball of snipers aside from spending the 12k mass to set up a t2 arty firebase or a t4 like a fatboy or a mega. Nerfing gun just pushes the game forward into static play earlier in those games.
I agree with the first point but not the latter, I believe that the cheap Gun Upgrade is one of the main things stopping units from contributing and unit advantages from being significant in the early game. Not that teamgames will magically be perfect after, but I think it's a start.