In a 1v1, gun with supporting tanks tend to be far better, because T2 is kinda slow : you need to build the PD, and once it's up, it can't kill a gun ACU, and it quickly dies to overcharge/arty/tanks.
Once there are 2 pds and more, the gun acu can be repelled, bu if the opponent chooses to run in, you won't have time to build so many !
Now on astro, players can assist each other, and multiple ACU can go gun or t2. Usually, people don't like to charge in with multiple gun so it oftens end up in a PD base war. But if you spot only one ennemy ACU forward, you can make gun to force him back and take few mexes.