Pretty sure pumpkin adds fermentables either way, though mashing probably adds more.
Edit: Hmmm... after some checking, it seems, if going by the weight of raw pumpkin, that sugar is about 1.5% by weight, and that starches are about 5% by weight. If using about 60oz of pumpkin for a 5gal batch (according to one I just took a look at), that means you've got the equivalent of less than 1oz of sugar, with another potential 3oz if mashed - though the accessibility won't be 100%, and you won't get 100% efficiency of the starches that ARE accessible.
Even still, for the sake of argument, let's say you can convert all that starch to an equal weight of fermentable sugar. So that gives you about 4oz of sugar total, in a 5gal batch. That's less than 2.5 gravity points, under *IDEAL* conditions.
And variability between different pumpkins and uncertainty of the efficiency of the whole process, it add anything, up to about 2 points.
So basically, unless you're using significantly MORE than a pound of pumpkin per gallon, you're really best off just figuring it MIGHT add a point or two, but not worrying about actually having to calculate it.