My use case is like this: I define in the kits environment an environment variable:

KIT_SYSROOT=%{SysRoot:FilePath}

The configured gdb and cmake are shellscripts that do something like

• source an environment file from the KIT_SYSROOT directory and then run
• ${KIT_SYSROOT}/usr/bin/cmake "$@"  or similar : gdb from the KIT_SYSROOT

At the moment (tested with QtCreator 5.0.1 and 6.0.0) that idea does not work. 

I have the impression that is an omission and it should be changed:
Set the kits environment for the cmake and gdb process.