The MCU settings page seems to differentiate kits only per board and OS, but not per toolchain. If a GCC kit is already present for a given board, it is impossible to create a kit for IAR unless the GCC kit is first removed.

Kits page - STM32F769i-discovery BareMetal has a kit for IAR but not for GCC:

MCU options - STM32F769i-discovery BareMetal GCC kit is considered already created. Impossible to create it.