How many people do you think download the application, then check the signature? Additionally, if you can spoof the download link on this HTTP page, you can also spoof the signature link, and provide a fake signature matching your malicious package.
Frankly, about the same number of people as the number checking the HTTPS certificates are as expected. GPG does have the advantage though that once the public key is known and trusted, the package can't be tampered with on the server. (Authenticode might also work, but then you're back to trusting all the CAs that Windows does.)
