Instructions
Open a text editor such as, for example, Notepad. Type the message that you wish to use as an MOTD.
Click on "File" and select "Save" in your text editor. Use the file browser to reach the main directory of your "Garry's Mod" installation. Typically, "Garry's Mod" is installed in "C:\Program Files\Steam\steamapps\steamusername\garrysmod\garrysmod," where "steamusername" is your Steam user name.
Navigate to the "addons\ulx" folder within your "Garry's Mod" folder, type "ulx_motd.txt" in the "file name" field and click on the "Save" button.
Open the "File" menu in Notepad again and select "Save." Navigate to the "data\ulx\" folder that is inside your main "Garry's Mod" folder, name the file "motd.txt" and click on "Save."