Forge / Fabric / OptiFine

When you choose a version on the download page, you can pick a loader at the same time. GPCL installs vanilla first, then installs the loader into its own version folder without overwriting vanilla.

How to install

1. Open the download page and choose a Minecraft version.
2. Under loader options, choose Forge, Fabric, or OptiFine, then the matching loader version.
3. Click download and wait for both vanilla and the loader to finish.
4. Go back to the launch page and start the version whose name includes the loader.

Download tasks are queued, so you can click several versions in a row; a running task can be cancelled, and the queue continues afterwards.

Which loader to choose

Loader	Good for	Watch out for
Forge	Large modpacks, long-established mods	Starts slower than Fabric; more versions to choose from
Fabric	Lightweight mods, new versions	Pick the Fabric Loader that matches the game version
OptiFine	Shaders, performance	Mixing it with other loaders may need compatibility mods

Forge

During installation GPCL only runs --installClient; it does not try GUI or server arguments. Installer output is turned into progress, for example "Installing Forge: Downloading library...".

The Java major version is checked beforehand; if it is below Forge's requirement you get an error right away instead of running it on an old Java anyway. The whole process waits at most 15 minutes; cancelling the download kills the installer process.

Fabric

GPCL fetches Fabric's version profile and libraries directly and does not rely on the installer.

"Install Fabric API as well" is checked by default; after Fabric is installed, the matching Fabric API goes into that version's mods. Uncheck it and nothing is installed; a failed Fabric API install only shows a notice and does not affect Fabric itself.

OptiFine

OptiFine installs silently and does not pop up its own graphical installer. The result lands in the current game directory, and custom game directories are handled correctly too.

Where mods go

Newly downloaded versions have version isolation enabled by default: the game directory points to versions/<version-name>, so mods, worlds, and game settings all follow that version.

Version status	Mods folder
Newly downloaded, isolated	versions/<version-name>/mods
Pre-existing, and the version folder has no mods / saves	mods under the game root

Open a version's "Version settings" on the launch page; the folder entries are computed from the actual isolation result:

• "Version folder" is always there;
• "Mods folder" appears when Forge / Fabric / NeoForge / Quilt is detected;
• "Shaderpacks folder" appears when OptiFine is detected.

To install mods into any folder, choose "Select folder" as the install target in mod downloads; it finds the mods folder under that directory.

It crashes after installing

1. Move all mods out of mods and start once with none.
2. If it starts, put back half at a time to find the problem mod.
3. Make sure the mod matches both the Minecraft version and the loader version - a Forge mod cannot simply be dropped into Fabric, and vice versa.
4. Missing dependencies also crash at startup; check the prerequisites listed under Mod downloads.

Mixing OptiFine with Forge / Fabric usually needs extra compatibility mods. If there is no matching compatibility mod, do not force it.

More troubleshooting steps are in Game crashes.