There has been some discussion in other threads but just to make it clear... There is a game breaking bug where terrorists and hostages are "non-existent" in the game but still visible on the map. For example in the first mission, on higher difficulties, you are unable to escort one hostage therefore not able to win the game. You can play on the easiest difficulty but you'll get stuck in mission 5 where one of the five hostages is invisible.

One way to fix this is to copy the "data" folder from an installation from the original CD.

There may be a better way to do this but this is how I got it to work:
1. Install the game from the original CD (there is a data folder on the CD but copying that does not seem to work).
If you bought the game here in Gog and you don´t own the CD just download it from a torrent site. IMO it's perfectly fine in this situation.
2. Install the Gog version.
3. Copy "data" folder from the original game folder to the gog folder.

I'm not 100% sure about this but if you install the original game AFTER you installed the Gog version then the Gog launcher will automatically use the new install destination. In this case you don't need to move the data folder, you only need to copy your campaigns and plans to the new install directory.
There's probably a config file somewhere in "my documents" or something in the registry that can change this?

Also plans and campaigns are located in /data/plans and /data/save.

Like I said, there may be a much easier way to fix this. Could be only one or a few files missing or broken in the Gog version that needs replacing.

My specs:
AMD FX-4350
Radeon 6870
8GB RAM
Windows 7

is this a bug that always occurs or is it intermittent? Seems like GoG should have fixed this some time ago as it's reported back in 2012. Luckily back in 98 or 99 my brother got me this as a gift so I still have the boxed game..oddly I had graphic issues back then and never got around to playing it...picked it up here so that it is all patched up..still had to use a DXwnd utility to make it work.

I'm not sure if everyone is having this problem with invisible characters but those who have have the exact same problems every single time (for example the hostage on the first floor in the first mission).

Please zip a copy of the data file, host it somewhare and link to it on the forum...gog could care less if you did this---its helping them...plus if people had the disk they would not even be looking to buy this title from gog... Thanks guys

I can confirm that replacing the GOG install data directory with a CD version's install data directory (after patching the latter to 1.04) does fix the invisible hostage bug. I will be filing a support ticket to inform GOG of the fact that their compilation is bugged and that this is the solution.