Yes it is possible to cover your gun in Realtree.
The method used by Tippmann is called water transfer, or hydro graphics. A thin layer of film is floted on the surface of a tank and the part is diped into the water.
The film has the design on it and the water pressure forces the film to conform to the shape of the part being dipped.
Alot of everyday common items that have a nifty design on them use a wter transfer to acheive the look.
What you would need to do is find a company that specializes in hydro graphics. Since Realtree is a copyrighted design it may take some time to find a company that will offer you the true look of Realtree. There are a good number of close aproximations though that simply do not bear the logo and are different in some small measure.