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I have downloaded and installed Freespace 1 from the GOG executable. However, there is no expansion. The manual only says Freespace : The Great War and the only missions are for the main campaign. In addition, there are no other download options for the expansion under My Account.

I have found out the name of the expansion is Silent Threat. Is this included in the GOG version? If it is how is it installed?

When you ran the GOG installer, did you go into options or anything? I ask because it is possible to uncheck installing the Silent Threat Campaign. Otherwise it should be there. The choice to select the Silent Threat Campaign should be in the Campaign room which is above the Ready Room.

You should also check out the Freespace Open Engine. It modernizes the graphics and compatibility for the game.
Post edited July 30, 2011 by crazy_dave
Looks like I unchecked the box when installing. Sometimes GOG wants to install other software like a PDF reader when I install a game. I probably thought silent threat was some type of crappy software rather than the expansion.

If you also own Freespace 2, I suggest installing the Freespace Open engine (see the dedicated thread) and playing the user-made campaign Silent Threat Reborn. It's basically a revamped Silent Threat with a better story and improved mission design. The original Silent Threat extension was rushed out and not very polished compared to FS1 and FS2 original campaigns, so Silent Threat Reborn will certainly give you a better experience.