There are a couple of manufacturers that make or have made modern functioning hand-cranked gatlings that are treated as Title 1 firearms, but they're not cheap;
Furr Arms made various scaled-down versions of the Gatling in calibers from 357 Mag down to sub-22 calibre versions;
http://www.lakesideguns.com/t1gallery/1874.html
Thunder Valley made full-size and miniaturized Gatlings;
http://www.ssgtactical.com/index.html?RiflesandShotguns
The Battery Gun Company makes full-size repros in several calibres;
http://thebatteryguncompany.com/index.html
BWE makes a half-scale version of the Gatling in 22 LR;
http://www.bwefirearms.com/
if you want a "non-gun" Gatling like the Predator version, Pipers Precision Products makes a version that runs off compressed air and shoots BBs;
http://www.pipersprecisionproducts.com/
or, if you've got access to a machine shop, you can build your own from plans. RG-G sells plans so you can make your own (videos at site)
http://www.gatlingguns.net/