The geometric aspects for the functionality of a molecule-based field effect transistor (FET) are analyzed. A computational study is performed on molecular models involving a well-defined conjugation plane coupled to gold-based electrodes through thiol bonding. Transport gating of the FET is shown to depend on a symmetry-breaking effect induced by the gating field. This effect is also related to the orientation of the field relative to the gold-thiol bonds, the molecular conjugation plane, and the overall symmetry of the device. First, it is found that the presence of a center of inversion in the bulk-coupled molecular system results in the cancellation of the transisting response. Second, a mirror plane of the molecule-bulk system, which includes the transport vector, will cancel the gating response to fields oriented perpendicular to that mirror plane. The symmetry properties are determined for the bulk contacted molecular junction.