Ah you're correct, it is gpg.exe, there is no gpg2.exe
The GUI is easier, but has it's own complexities. There are two different applications, GPA and Kleopatra, and if I recall correctly you also need to sign the Qubes key yourself otherwise you will get an error, so it also involves creating your own keypair.
@Maria
Try this. Open GPA, import your downloaded master key. Check if the fingerprint is correct. Right click set owner trust and set it to ultimate. Then download the release signing key and also import it. Download the Qubes iso and signature. Right click the signature and go to More GpgEx options, and click Verify. If the verify gives you an error, try command line:
cd C:\Program Files (x86)\gnupg\bin
Then use gpg to verify(change path to correct download folder, you can right-click the signature file, click properties, and copy and paste the file path):
gpg.exe --verify C:\Users\*your username*\Downloads\Qubes-R4.0.3-x86_64.iso.asc