Go to the page [samllea1.github.io]
The page should look like this:
Choose the image you want to be as the nextbot. Then choose the name for your nextbot. Select if you want a custom sound or remove the default one.
Click "Export Mod" this will save a .zip file.
Open that .zip

Then go to C:\Users\[PC-USERNAME]\AppData\LocalLow\F2Games\GoreBox\Mods (replace [PC-USERNAME] with your actual PC name) and move the folder inside the .zip to the directory i mentioned.

Open GoreBox and go to Addons then Mods and you'll see the mod/replacement you did on the site. Click on it and Enable it.

Congratulations! You got a custom nextbot in GoreBox!