A GDCB file is a file encrypted by the GandCrab virus, a trojan horse utilized by cybercriminals. It contains a user's file, such as a .PDF or .WMV file, encrypted with the Salsa20 algorithm. GDCB files became prevalent in January 2018.

The GandCrab virus is ransomware, where the purpose of the virus is to take your computer files hostage and force you to pay the perpetrator (typically via DASH currency) to unlock your files. The GandCrab V4 virus scrambles, renames, and encrypts the user's files. It then generates a .TXT ransom note (GDCB-DECRYPT.txt) informing you of the file encryption and what you need to do to recover your files. The Bitdefender GandCrab Decrypt Tool may be able to decrypt GDCB files and it is available for free. Also, if you backup your files you can execute a full system restore. Otherwise, the only known way to decrypt your infected files is to pay the perpetrator for the private key and decrypt program.