Leave the 'Start in' field as it is. It should have the full path to the directory where the game was installed. If you didn't change it from the defaults set in the GOG installer then it would probably be:

"C:\GOG Games\Red Baron Pack\Red Baron 3D\"

The -window should rather be added to the end of the Target field, e.g.:

"C:\GOG Games\Red Baron Pack\Red Baron 3D\Baron.exe" -window

Note that this isn't passing -window to the GOGLauncher.exe anymore but directly to the game executable. I suspect that the GOGLauncher.exe is a wrapper that GOG made to smooth over some problems with this game on modern systems but it doesn't work for me and I guess some other people here too!

Note that this is just to get the game to run at all. If you follow Wailwulf original instructions, you'll see that he used this method just to get the game started so that he could switch to using the Glide renderer from the default Direct3D one. Once that's been changed then you can quit the game as he said, restore the Target back to the original (or rather just remove the -window flag; I still leave it pointing at baron.exe rather than the GOGLauncher) and start it up again.

If now you see that the game main menu is in a window, you can use Alt+Enter to put it in to full screen. The game seems to be using a set resolution for the menu and a separate resolution, that you can specify in the Preferences window, for the main game. I generally set the main game resolution to the maximum which nGlide then scales to the entire screen automatically. You should only have to do this once. Every time you restart the game after this it should all be full screen.

If you do put the main menu in to full screen as above, you will still most likely see a brief flicker of your desktop when you switch to flight mode but it's not a problem, just slightly unsightly. You will also probably see the 3dfx logo the first time you switch to flight mode in each session (although you can configure nGlide to not do this with the little configuration utility it comes with). Personally I like to leave this on for nostalgia's sake :-)

If you're still having problems then there is something else you can try but this is a little more tricky so do this at your own risk! I install nGlide directly and delete the glide related dlls from each GOG install. That would typically be glide.dll, glide2x.dll and glide3x.dll. This will force Windows to search for the system wide install of a glide wrapper on your machine (in C:\Windows\SysWOW64 on a Windows 7 x64 system).

This is only really necessary if the GOG installer hasn't been updated with a recent Glide wrapper (and of course, only applies to games which used Glide in the first place). I didn't check if RB3D was using a recent version - I just routinely do this with Glide games as I know the latest version of nGlide is working well on my system. Of course, you only need to do this if you're still having problems after following Wailwulf original advice. You can always re-install the game if it makes things worse - although it's unlikely to in my experience as I've done this with every Glide game on GOG so far because a couple of them were using quite old Glide wrappers when I last downloaded them (I-War being the most obvious).

Hope something here helps!