Instructions
Open your favorite Web browser and download the HLDS Update tool (see Resources). Save the tool on your hard disk drive and double-click on it to start the installation. Follow the simple instructions you see on your screen to finish installing the HLDS Update tool.
Go to the "Start" menu, click once inside the "Run" box and type "cmd" (without the quotes). Press "Enter" to start the Windows Command Prompt tool. Type "cd c:/srcds" (without the quotes) in the Command Prompt and press "Enter" to navigate to the folder where you installed the HLDS update.
Type "hldsupdatetool.exe -command update -game garrysmod -dir" in the Command Prompt and press "Enter" to update your Garry's Mod version.
Open a text editor, such as Notepad, and type "srcds.exe -console -game garrysmod -port 27015 +maxplayers 6 +map gm_construct +sv_gamemode sandbox +sv_defaultgamemode sandbox" (without the quotes). Replace "27015" with the port number you want to start the server on. Replace "6" with the maximum number of players you want on your server. Replace "gm_construct" with the map you want to run when you start the server.
Go to the "File" menu and select the "Save As" option. Select "All Files" in the "File type" drop-down box and enter "startserver.bat" in the "File name" text box. Use the built-in file browser to navigate to the "garrysmod/orangebox" directory and click on the "Save" button.
Open Garry's Mod and create a new server. You are now able to find your server in the "Internet servers" list.