Hey Junejune, were you able to resolve your problem? I've been having the same issue.
I'm using Firefox with a download manager. This problem happens even with small programs so it's doubtful that any files are actually corrupted during the downloading.
I've extracted the files with both 7-Zip and Peazip into a directory.
I have disabled AVG and Comodo Firewall during both the download and installation process.
I run the Setup file as Administrator and the software is always successfully activated by GOTD and immediately afterwards I get the corrupt file message.
I've repeated the entire process four or more times on several different offers.
Any help would be appreciated. I've never had any issues with GOTD before the introduction of .gcd's.
Thanks