Optionally, click the Modpack settings link in the upper-right to select a specific version and the installation path.Make sure the pack named "GT New Horizons" is selected in the results list. In the launcher, go to the Modpacks tab and enter "GT New Horizons" in the search box.Under settings - Java, make sure you are using a version of 64-bit Java 8.Download a MMC version of the pack from and drag it into the MultiMC window to create the instance.If you are on Windows 10 or 11, make sure you are not installing it in a OneDrive folder! If you are experiencing issues with fullscreen mode, set a resolution in your launcher and check other settings in the config/lwjgl3ify.cfg config file. You only want the Java args from that page, nothing else. If you are using MultiMC you also need to add the additional Java arguments from here.In the case of Java 17 make sure you are updated to at least 17.0.6, older variants can cause start-up errors. Under settings - Java, make sure you are using a new version of Java, best 64-bit Java 17 or 64-bit Java 20.
Drag it into the MultiMC/Prism window to create the instance.