The file d2gdi.dll is part of Diablo 2 from Blizzard Entertainment. This file allows the program to be run in windowed mode. Diablo 2 is a hack and slash role-playing game released in 2000. It was extremely popular, in part because of access to an on-line component that allows multi-player gaming. Gamers like to have control over their programs, and one option is to run the game in a window instead of full screen.
If this dynamic link library is missing, the program won’t be able to run in a window and will produce an error instead. Because the file is in the dll format, it is also possible, although unlikely, that other programs will be accessing the code in the file. If so, those programs will also produce an error when you attempt to run them.
Fixing this error may require a reinstallation of Diablo 2, or you can try running ZookaWare PC Cleaner, the registry cleaner. ZookaWare PC Cleaner will fix the errors in your registry that result from moved or missing files, and get you up and running again quickly.