After the personal gateway is installed, “C:\Program Files\Power BI Personal Gateway\version\Configurator\GWConfig.exe” is launched to configure the Power BI account and domain credential. Once you setup the gateway using a Power BI login account, you are not allowed to change the Power BI account, unless you uninstall the personal gateway and install again.
So, does this issue exist in a fresh newly installed Power BI Personal Gateway? If not, please uninstall it and then reinstall.