Automation vs control
MultiMC's design is "do nothing the user didn't ask for." That means you set the Java path, you pick the version, you manage mods, you update them. Power users love this. New players bounce.
GDLauncher takes the opposite approach: detect what each instance needs, install it, keep it updated, but expose all the same knobs in instance settings if you want to override anything. The defaults work; the controls are still there.
Modpack handling
MultiMC has a built-in Modrinth browser, but no CurseForge integration. To play CurseForge packs you'd need to import them manually as zip files, or use third-party tools to fetch the manifest. GDLauncher's browser shows CurseForge and Modrinth side by side, with one-click installs on both.
The legacy
MultiMC hasn't shipped a new release since December 2021; the project's energy has effectively moved into Prism Launcher and other forks. If you've used MultiMC for years and want a more modern UI without losing the workflow, Prism is the natural upgrade path; GDLauncher is the bigger jump (more automation, fewer manual steps). Try both and pick the model that fits how you actually use a launcher.
Cloud Instance Sharing
Sharing a setup with a friend in MultiMC means exporting the instance to a zip and handing the file over. That works, but it's a file you have to host somewhere, and the recipient has to import it the same way. GDLauncher's Cloud Instance Sharing replaces that with a short code: paste it, the launcher pulls the snapshot from the GDL service, and mods re-download from their original CDNs. One code, mixed CurseForge + Modrinth content in the same share, no zip file to pass around.
The verdict
MultiMC is a great choice if you want a tiny, hyper-flexible launcher and are happy doing your own Java setup, mod management, and updates. GDLauncher is for players who'd rather have those things handled automatically, auto Java, auto updates, one-click installs, Cloud Instance Sharing, and server management, without sacrificing the multi-instance workflow MultiMC pioneered.