You don't seem to realize that the software license means no warranty, but that does not preclude anyone from giving support and warranty on the product.
So yes, GPL'ed software may be used on critical applications the same way as every other piece of hardware and software: via special contracts.
I realise that. Hell, I've done that. As a licensed engineer, I would just be very concerned about contributing to an open source project that controlled critical systems in cars. That sounds like all the liability of my actual employment, but none of the pay.
Unauthorized use of your code or product for uncovered applications is a problem no matter if the source code is open or not. Companies exclude themselves from such liability as a matter of policy, demanding special contracts on these cases.
If you get any IC datasheet, or even an application-specific brochure such as (1), you will see a notice saying explicitly that.
So yes, GPL'ed software may be used on critical applications the same way as every other piece of hardware and software: via special contracts.