Projecting on drywall will work in the sense that you will see a picture, but you won't be getting the best from your projector.
White screen materials are a much truer white than drywall, so you don't end up with color shift. Also, the surface of a screen will be more uniform than drywall. Your eye will be forever drawn to any imperfection in the drywall once you notice it. Drywall also does not benefit from the sophisticated optical coatings you get with higher gain screen materials.
If budget is a big concern, there are many inexpensive manual pull-down screens such as the Da-Lite Model B that will give you excellent performance at a low cost. If you are really trying to cut costs, there is also "Screen Goo" paint, which I admit I'm not all that familiar with.
One temporary advantage to projecting onto drywall is that it gives you time to play around with the size of the projection, using the projector's zoom. If you already have decided on the optimal projection dimensions for your viewing room and seating distance, then a good professionaly made (or DIY) screen would, as Marc said, give you a better picture.
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