Black Mesa in Garry's Mod?

Is there any way to get maps, models, props, etc. into Gmod?

Google is your friend.

So are the people here.

No.

Founded in 2004, Leakfree.org became one of the first online communities dedicated to Valve’s Source engine development. It is more famously known for the formation of Black Mesa: Source under the 'Leakfree Modification Team' handle in September 2004.