The older versions of IAR Embedded Workbench do not support EFM32GG11. Fairly recent versions of IAR EW support EFM32GG11. For example, 7.80.4 does not support EFM32GG11. You can check this by creating a new project in IAR. Right click the project and select Options. Under the General Options category, select Device. Click the button next to the text box to see a list of Semiconductor devices manufacturers.
Navigate to SiliconLaboratories to see if EFM32GG11 is listed in the available devices. If it is not, then the EW is not compatible.
IAR Embedded Workbench version 8.20.1 supports EFM32GG11. You can use the option to install CMSIS-Pack for EFM32GG11 here -
The CMSIS-Pack installer for EFM32GG11 is available on the website by using this URL - https://www.silabs.com/documents/public/cmsis-packs/SiliconLabs.EFM32GG11B_DFP.5.4.0.pack