What can you tell me about this handgun? a Six shooter, 2 1/2" barrel, very much alike the colt detective ...

Is it a real mauser? Or a Renato Gamba´s replica?

I know Smith and Wesson made some that were marked Walther for the German market.
I think you have a Colt that is marked Mauser. The side of the barrel where the Mauser marking is seems to have been machined flat. On a Colt it was rounded. So Mauser, or Colt for Mauser, might have machined the barrel "flat" and remarked it. I would contact Colt Manufacturing and see if they can shed some light on it.
Except for the markings and flat side of barrel that is a Colt right down to the medallions on the grips.
If you are not able to contact Colt, let me know and will contact them for you. I am interested in this as well.