Here’s de_star, which was previously mentioned.
Download here: http://www.gamebanana.com/maps/18142
This one looks good, too. It’s called “dm_pinnacles”
Another great one, dm_neon, from the same people who made dm_cloudcity
This one, dm_3006, is supposed to be like the underbelly of a giant futuristic city, below all the bright neon signs, etc.
And here’s another awesome looking one, dm_tech
EDIT: I don’t know if this map will work in Gmod, but it looks awesome and futuristic.