This is for anyone who may be confused on what to do with the downloads in the description like I was. After you download the file from MEGA, you'll want to right click and use the "extract files" option to a folder of your choosing. Once that's done, click on the files you've extracted an choose the Gmod Fallout4_Synths folder. Make sure it's the right one by clicking and seeing if it leads to two other folders called models and materials. Drop it into the addons part of your Gmod files by going through file explorer.

Steamlibrary > steamapps > common > GarrysMod > garrysmod > addons