In Bluetooth SDK v2.3 Bluetooth projects do not include Gecko Bootloader by default.
If you want to add Gecko bootloader do the followings:
Note: bootloader-uart-bgapi-combined.s37 contains the first+second stage of the Gecko Bootloader, while bootloader-uart-bgapi.* contains only the second stage. The first+second stage is needed when flashing the bootloader the first time, while the second stage is needed when upgrading the bootloader (either by flashing or by DFU).
Note: commander.exe can be found in: C:\SiliconLabs\SimplicityStudio\v4\developer\adapt