Download the Derived DarkRP Gamemode. Extract the zip file into your /garrysmod/gamemodes folder. You can do this either via SFTP or the panel's web file browser. In the end, your gamemodes folder should look something like this:
Open the derivedrp.txt file in the derivedrp folder. Change the title, e.g. to StarWarsRP or MilitaryRP.
Save and close the file. Then rename it, e.g. to starwarsrp.txt or militaryrp.txt. (lower case only)
Now open the gamemode folder. You will find 2 files there: cl_init.lua and init.lua. Open both and change the GM.Name, e.g. to StarWarsRP or MilitaryRP.
Now all you have to do is set the Gamemode in the Startup tab and restart your server.