You can use for example .TGAs (or equivalent .DDS format) with alpha channel for texture and load it with appropriate flags.
You can also separate all your window polygons to a separate object and use entity alpha to get the desired level of transparency.
GIF won't work at all, so forget it.
You can also separate all your window polygons to a separate object and use entity alpha to get the desired level of transparency.
GIF won't work at all, so forget it.