Using a previously developed formalism based on the generator coordinate method, the longitudinal- and transverse-excitation form factors for the giant dipole resonance are obtained. Although microscopically established, that approach can provide analytic and compact expressions for some selected closed-shell nuclei. Our calculations are then compared with the phenomenological Goldhaber-Teller isospin-mode result and the experimental data. From our method, we derive restricted RPA-results (quadratic approximation) which are analysed.