I mean aluminium, but will likely affect aluminium containing alloys in the same manner anything that is too caustic (high pH, typically 12 and up) will damage Aluminium.
It's just gonna be the outer surface (unless you leave it soaking for days) and this depends on what is in the cleaning stuff. But if it's anodised, then it may well strip the finish off.
This isn't to say that this is the case with Gunk, it could simply be that the parts were not treated correctly during manufacture and any cleaner would have done the same job. All I'm pointing out is that you should check any cleaners bottle or website to determine if there are going to be any compatability issues before you use it realisticlly, if it's a general cleaner yo should be fine.
We had an aluminium rotor for a piece of kit at work that was anodised and costs about £5,000 a pop. Some bright spark used the usual cleaner (which happens to be v. cautic) and took the anodising straight off. It still works thankfully, but even after we told everyone, someone else did the same thing with the spare one. Just something to keep in mind that all.
Snip from Gunk's product info:
PRODUCT LIMITATIONS:
Will not prevent rubber cracks due to age and ozone attack. Avoid contact with
aluminum or magnesium wheels, painted surfaces, glass and non-rubber surfaces. If splashed on these
surfaces, rinse off over spray promptly to prevent damage.
Based on this, I'd watch all unsealed aluminium as well (SV swing arm) for instance, you'll have a finite time so if you was off imediately you'll be OK I would think. FWIW, it's basically a 5% caustic solution. I go terps or parrifin on engines etc.
Ste