Greaziest: The Hi-Res pack utilizes the program eDuke32 which is a port of the game engine Duke3D used to originally run the game, you don't have to deal with DOSbox at all.
If you installed Duke3D into its default folder using the GOG installer then the original game files are located at
C:\GOG Games\Duke Nukem 3D If you downloaded the
FULL RELEASE (SFX) version of the Hi-Res pack then it will also install the shareware version of duke3d.grp which lets you play the same levels the shareware version allowed you to. If you attempted to install the Hi-Res pack into the default GOG install location of
C:\GOG Games\Duke Nukem 3D please be aware that it will create a folder to install itself into called Duke3D....
C:\GOG Games\Duke Nukem 3D\Duke3D will be the location where the Hi-Res files are installed at. In order to get eDuke3D to run the Atomic Edition of Duke3D you need to go back to your installation of the original Duke3D game and find the file called
duke3d.grp, the file size will be ~40MB. Copy that file and place it into the folder where the Hi-Res pack which would be
C:\GOG Games\Duke Nukem 3D\Duke3D IF you installed it into the same folder as the original GOG installer installed the original Duke3D files.
*NOTE:* I've had to make a lot of assumptions based on the limited information you've included, the folder locations will be different if you installed Duke3D or the Hi-Res pack into different areas on your hard drive. But no matter where the Hi-Res pack is installed, the most important file will be
duke3d.grp located in the folder where Duke3D atomic edition is installed, which needs to be placed in the Hi-Res pack folder with eduke32.exe
TL;DR: Copy the Atomic Edition duke3d.grp file and replace the duke3d.grp included with the full install of the Hi-Res pack. When you run eduke32.exe it will now give you an option to play "Duke Nukem 3D: Atomic Edition" *edit* grammer
