From Runic Games Wiki
For Steam users
GUTS is available through the Steam client. To obtain it, do the following:
- From the Steam client, open the Library menu and choose Tools.
- In the list of tools, locate Torchlight II GUTS and double-click it to begin installation. Once installed, you'll be able to run GUTS from here.
For non-Steam users
GUTS was automatically patched into Torchlight II for non-Steam users. You can run it by following these steps:
- Launch Torchlight II and make sure it's patched up to the latest release (1.22.x.x).
- Click Launch GUTS underneath the Play button as shown below: