MD5 check
First of all, enable it. Go to server.ini and set USE_MD5 to 1. Then, get the program Chaos MD5. That program will generate the MD5 hash of your Gunz.exe. You can download it at
http://www.elgorithms.com/downloads/chaosmd5.php. After you've downloaded it, extract it and browse to /unicode. After that, doubleclick ChaosMD5.exe. In the window that appears, hit the small folder and select your Gunz.exe. Then, hit Generate MD5. After a few seconds, a weird "word" will show up. That is your MD5 hash. Copy and paste it into MD5HashValue.txt. For example, the MD5 of my edited Gunz.exe (old one ofc.) is D190E5AAFC631CF493EA551607DB53FB. After you've copied the hash into MD5HashValue.txt you're not ready yet. You'll need to put spaces after each combo of 2. So, D190E5AAFC631CF493EA551607DB53FB will end up as D1 90 E5 AA FC 63 1C F4 93 EA 55 16 07 DB 53 FB. Now save your MD5HashValue.txt and you're done! (After you restarted the MatchServer ofcourse)
After you've done the steps above, you should only be able to connect with the Gunz.exe you provided the MD5 hash of. Any other Gunz.exe will result into Cannot Access Server.