There is no set dihedral on the GP skybolt. It is built into the wing during construction. the top wing is flat, and the bottom wing sections are joined while the wing is ON the plane attached to the "I" struts. Just follow the steps and it will all work out fine.

The plane drops like a rock because of it's thin wings and beefy weight, which is also what makes it so fast and maneuverable. It has nothing to do with the dihedral.