You'll need a Bukkit/Spigot/Paper server first. Download the Bukkit plugin from [releases](https://git.exmods.org/NorbiPeti/GCMC/releases) and put it in the server's plugins directory.
Then run `/export <x1> <y1> <z1> <x2> <y2> <z2>` on the server with the starting and ending coordinates of the area you want to export.
After that, copy the `result.json` from the server folder into Techblox's folder.
Then, using the Techblox mod, do `importWorld "result.json"` in the GC console (opened by pressing the button near right shift by default) to import the Minecraft blocks.
Open the game log (and scroll to the end) to see the blocks that the mod doesn't know how to place in GC. You can add block mappings to the BlockTypes.ini file in Techblox's folder following the format (Minecraft block goes in brackets, the rest are Techblox block settings, color darkness goes 0-9).