I believe you would need to contact GOG support for that information. I don't recall seeing it available on the site although I have not really been looking for it.
I don't know your particular situation but out of several dozens of games that I have purchased and installed from GOG, none have given me problems, viruses or spyware. I scan the download with my AV and Spybot and, along with passing the integrity check, I feel confident the file is legit.
Your situation may warrant a closer examination so I would suggest asking GOG support if you need more confidence in the file.
Edit: If you are running Vista / Win 7 than be sure to right-click the file and click "Run as an administrator" from the resulting dialog. Not doing so can cause a partial install of the game. I also suggest turning off UAC during the install and turning it back on after-wards. If you are runing a 64 bit OS I would suggest installing in a directory outside of Program Files / Program Files (x64). After installing, right-click the game shortcut, click properties, click the Compatibility tab and check the "Run this program as an administrator" check box. The game may not run properly if you don't do some or all of the above . . . hope this helps . . .=)

No, I do not encounter any problem. I simply want to know the MD5 checksum so that I can really ensure my copy is not corrupted (since my first download was corrupted, and this is my second download). It does pass the self-integrity check, but I guess I am just being thorough. That's all. Not a problem actually.
Alright, I'll contact the GoG Support. Thanks.

Here is the MD5 I came up with for the setup file: f8392a03dd78739ede7169ca77ddfd8d

I sent the inquiry to GoG Support two days ago, and until now they have not replied yet. But my installer MD5 checksum matches with carlosjuero's. Thanks, carlosjuero :-)