It is nearly impossible to price the worth of a Gulbransen Piano without the aid of a professional familiar with them. A good way to try and estimate its worth is to look through sale and auction sites to see how much others are going for.
1 Additional Answer
The worth depends on quiet a few factors like year it was manufactured and the condition the piano is in. You will need to have it inspected by a trained piano technician and appraised by a dealer.