I've set up the new GateKeeper module.  When I get the order email and click on the link (or paste it into a browser) it correctly downloads a zip file but the zip file is "corrupted".  I did a binary compare of the actual file and the downloaded file and the difference is that the downloaded file has the HTML for a web page appended to the end of it.  That is to say that he entire ZIP is there and correct, but the file contains a "suffix" of data that is raw HTML.  It looks like it's probably a page on my site (I didn't try to scrape it to a separaste file to verify).

Any ideas on how to prevent this behavior?

I advise you to use programmed way of recovering .zip file.

Tool below must help you anyway

Fix zip file

program provides you recovery of zip archives next cases: ZIP file you have been downloading for so long is broken - it's your case, corrupt and cannot be opened and others.