Step 1: Make sure Half-Life 2: Episode Two is fully installed
Step 2: Launch Garry's Mod
Step 3: Once loaded click the little controller in the bottom right corner, this will bring up the mounting menu.
Step 4: Find Half-Life 2: Episode Two
Step 5: Click the small box to the right of the game

Now you can most likely play Cosmonaut without errors or missing textures.



[If you still see errors and missing textures try reloading the game after mounting Half-Life 2: Episode Two]

Edit: Don't Forget to get the two other addons needed for the map. They aren't optional they are needed.