GatherOsState.exe is a program in Windows which allows a computer to Clean upgrade to latest version of Windows without going through intermediary Windows Key Upgrade Process. In case of any issue, if you get an error code 0XC1900126 (MOSETUP_E_GATHER_OS_STATE_SIGNATURE), then there is a problem with signature.

The GatherOsState executable has an invalid signature.

GatherOsState.exe is available in ISO files you either download or have created using the Media creation tool. In case of signature issue, you may want to try to download the ISO file again, and repeat the process. For some reason, the GatherOsState.exe has an issue from that ISO file.