"anti-grav" entities

[illiterati]

I haven't had a great deal of time to look over tutorials that may explain this, I've been busy enough just building a level and gettings a grip on the (new) basics of lighting -- cubemaps, etc -- but eventually I hope to include something like the massive fan from Half-life Uno that propelled you toward the ceiling (on a somewhat smaller scale).

If anyone know whether and/or how something like that is possible (with the HL2DM FGD), I'd appreciate a heads-up.