From https://www.gog.com/forum/outlaws_a_handful_of_missions/invisible_dynamite :

1. Download nGlide 1.05 from here: http://www.zeus-software.com/downloads/nglide
2. Open the nGlide105_setup.exe file in 7-zip (other unzippers may work, but I know 7-zip will)
3. Copy the all the files from nGlide105_setup.exe except $PLUGINSDIR to your Outlaws game folder, replacing all files.
4. (Optional) Run nglide_config.exe to set whatever options you want
5. Run Outlaws in 3dfx mode (Direct3D mode still doesn't render 3D objects

Credit to steam user "theweis" for the original steam forums post this is based on, and GoG.com users for discovering this fix.