The GROUND BRANCH Mod Kit is available (only) via the Epic game launcher (because of Unreal Engine licensing restrictions, since you ask). It is available via this link, for example: https://store.epicgames.com/en-US/download
Search for GROUND BRANCH and you should be able to find the Mod Kit, shown here in a player library:
Figure 1. Mod Kit installed in your library
Once you ‘buy’ the Mod Kit (it is free), it should start downloading to your Epic game library. It is launched via the Epic game library in the usual fashion.
Once you have installed the Mod Kit, you can start modding: Mod Kit overview.
At the time of writing, currently only weapon modding is supported. See the weapon modding guide.