Many heavy weapons can be used as ad-hoc anti-aircraft weapons but the likely hood of hitting is usually quite low. The guards dedicated AA weapons are numerous.
Sabre Platforms with twin linked auto-cannons (or heavy bolters, lascannon, or quad mounted heavy stubbers) are basically the lightest, the requivalent of that weapon that Routa-maa linked.
Then you have the Icarus Lascannon and Quad-guns (quad mounted auto-cannon very much like the hydra) which are usually part or more permanent fortifications such as teh Aegis defence line and Bastion. These have more sophisticated targetting systems.
Then you have mobile AA. The Hyrdra flak tanks with it's quad long-barrled autocannon and advanced targetting, and the Manticore when armed with heat seeking Sky Eagle rockets.
Both the Hydra Flak cannons and Manticore rocket launchers are also found as towed blased weapon system plaforms.
The largest Imperial Guard standard anti-air craft weapon is in the super heavy Praetor armoured assault launcher using anti-air missiles:
I think most of the previous post has summed it up but just to reiterate that the Hydra mounts a pair of long-barrelled twin-linked autocannon. In tabletop terms it increases the range and gives them a re-roll. It alos makes it one of the most brutal anti-infantry weapons mounted on an imperial vehicle until the Punisher canon for the leman russ was brought out.
Similarly the Manticore is a multi-role missile launcher, but going on the size of the missiles and their high strenght and Ap in the tabletop i would use stats far better than that of the hunter-killer missiles (which are just suped-up krak missiles with guidance systems).
"Only the insane have the strength enough to prosper. Only those who prosper may truly judge what is sane."
I just remembered that in an Epic 40K ofshoot called Aeronautica they models for a heavy AA gun. Very much like most WWII AAA.
I guess that's about it. You make up stats for most of it but I can't be bothered doing it, not knowing what you actually need it for. Some-one mentioned Flak missiles for missile launchers as your man portable option.
The only other thing I can think of saying is that it's a big galaxy, plenty of stuff out there. Locally made vehicles with any of the AA weapon combos, rarely used Chimera, Leman Russ or any other IG vehicle based AA weapon platform and I'm sure multi-multilasers would work fine, maybe in fixed emplacements or on mechanicus vehicles.