Is 3DMigoto GIMI (Model Importer) free?

Yes. 3DMigoto GIMI (Model Importer) is released under Open Source / MIT and costs nothing. Neither it nor Genshin Impact has any paid content.

What do I need to run 3DMigoto GIMI (Model Importer)?

Genshin Impact, which is itself free. 3DMigoto GIMI (Model Importer) is an add-on and will not run standalone.

The PlayBound Launcher installs it in one click from Genshin Impact's mods list. You can also install it by hand — full steps are on this page.

Roughly 35 MB, on top of the base Genshin Impact installation.

Yes. The source is published at github.com/SilentNightSound/GI-Model-Importer under Open Source / MIT.

3DMigoto GIMI (Model Importer) — Free Genshin Impact Mod · PlayBound