1. paste the file in the Advdupe2 folder.
SteamLibrary\steamapps\common\GarrysMod\garrysmod\data\advdupe2
2. Start Gmod and join the server. (of you are in the server already click the refresh button in the AdvDupe tool should be near the top
3. Select the file and paste it.
4. enjoy the base design.