I think the balance for Base shooting is very good.
A base is basically a bunch of problems stuffed together that require certain tools to be solved. A mammoth covered by a SAM and a flak is something Koegy would let others kill, but for those who need to kill that - they won't send in cobras. For this kind of problem Mammoth are the best choice. For another problem Paladins are best, for a third one you want cobras and on another one Avatars are the best way to break the problem. For medium and difficult bases GDI is usually a bit stronger than NOD. On one hand because they get some of the strong tools to use out of the box or way earlier than NOD and on the other hand, because GDIs units are more based on 2.5 range which lets you outmaneuver the defense you hit more easily and you can cause more damage with the same offense level. But yes - on easy bases or on very special problems NOD is stronger, simply because they can avoid some unnecessary structure killing and can bypass parts of the defense more easily.
The biggest problem is OP farming. NOD has all the tools for farming efficiently. And GDI players need to be super active and watch for those 25% easy OPs that they can kill with hardly any RT efford.
In PVP the difference is not that big. I don't agree that the NOD defense is stronger. Stunning vehicles is pretty strong and stunning infantry as well. NOD has the stronger Taxi offenses, GDI has the much stronger anti taxi defenses. GDI has stronger flaks, has stronger anti vehicle infantry and while the anti air units can only hit in 1.5 range they can damage significantly in 2.5 range with the upgrade. Without the upgrades the GDI defense is weaker, but after the upgrades it is a bit stronger.
On offense GDI vs NOD is not that poorly matched - you have an AoE structure killer that can easily kill any front structures, you have mammoth with great trample damage and 2.5 range. And the damage vs inf is not bad at all. Yes the avatar has about 20% more hitpoints and 20% more trample damage and that surely helps.
For GDI air units - if you build a line of orcas and stick kodiaks in between for the shield they can cause a lot of damage. And they wait and kill the units and don't just fly over them.
So the only part in PVP where I see GDIs with a clear disadvantage are Taxi offenses.
I would suggest to not buff or nerf the commonly used units for either faction for PVP, but rather change the underused units. The recent artillery changes are a step in the right direction.
And I think if you buff underused defensive units you could address the few disadvantages that GDI actually has in PVP.
1. MG Nests: Give them an air stun upgrade like forgotten have it
2. Guardians/Reckoners: You could let the units inside actually shoot as well - stealth militant hitting a guardian would result in the guardian and the rifleman inside to shoot as well.
3. Attack Bikes: let them actually move through units
4. Predator/Scorpions: Give them an additional close range defense that damages ground units in 1.5 range. Simply to incentivise the use of 2.5 range units against those tanks.
5. Artilleries: They need either more damage or less defense points consumption.
These changes would help GDI slightly in defense, since they have the better units to hit vehicles. And it will change the defenses from mainly infantry based + a few structures to a well rounded mix.