You say "stencil piano" as if it were struck with the plague. There's nothing wrong with a "stencil piano". All that means is that it is sold under a different name than that of the company that built it. Steinway's Boston and Essex lines are "stencil pianos". So are a lot of other pianos out there, many of which are just as good as those sold under the factory name.
Second, anyone trying to scare you about the possibility that something as old as a Gulbransen might be a "stencil piano" is being ridiculous. Does the piano do what the seller said it would do? Do you like the piano? Will it stay in tune? Does the action work properly? Other than the concern you have over the name, do you like the piano and are you satisfied with it? If you are, then stop wasting energy worrying about who built it. It's doing exactly what you wanted it to do.
If the piano has the name Gulbransen on it, then it's a Gulbransen. Just because you can't find the serial number doesn't make it a "stencil". Even "stencil pianos" have serial numbers. You just haven't located it yet.
There's no conspiracy going on - no one is taking the time to remove serial numbers from mediocre pianos to hide the maker of it only to put the name of another mediocre piano maker on the piano. Since you just got the piano, you're should be having it tuned within the next week or two. Just ask the technician to locate the serial number for you. If it isn't in the normal location on the plate, he can probably find it on one of the case parts somewhere. After he/she has tuned and serviced your piano, arrange with the tech to have your piano put on a regular 6 month schedule of tuning. This is what is required to properly maintain a piano, and anyone as concerned as you are about the possibility of your Gulbransen not being a real Gulbransen should certainly be willing to do what is required to properly maintain your investment.
Hope that helps you.
Life isn't measured by the breaths you take. Life is measured by the things that left you breathless