Team Fortress 2: Gold Rush is a modification for Team Fortress 2 and, as such, it requires the asset of the main game to function. However, it does not require all of them. If you’re not planning on going back to playing regular TF2, you can safely remove everything that’s not needed by Gold Rush.

Before proceeding, make sure that you have launched TF2 at least once after installing it. Then, go to the directory where the game is installed. By default, it’s C:⁠\Program Files (x86)\Steam\steamapps\common\Team Fortress 2, if you’re on Windows, and ~/.local/share/Steam/steamapps/common/Team Fortress 2, if you’re on Linux. Once there, delete everything except for the following files and directories:
In place of * should be either a three-digit number starting with 0, or dir. For example, you must keep files from tf2_misc_000.vpk to tf2_misc_026.vpk, and tf2_misc_dir.vpk.

The biggest offender in terms of wasted disk space is the \tf\maps\ folder, taking up a whopping 10 GB at the time of writing.

Unfortunately, you will have to repeat this process every time Team Fortress 2 updates. To make this less unexpected, disable the automatic updates by selecting TF2 in your Steam Library, opening its Properties, switching to the Updates tab, and selecting “Wait until I launch the game” in the Automatic Updates drop-down list.