Dear Christian, the bandgap by definition is the electronic transition between states at the lowest conduction and the highest valence band. Therefore, in the optical absorption spectrum, the only contribution to the bandgap should be the lowest one you can observe. Any other contribution can be due to other processes including other electronic transitions or even phonon-induced sidebands. Care must be taken in regards to the model, however, because the Tauc plot was developed for 3D semiconductors, not for 2D semiconductors (please check Mark Fox's book Optical properties of solids). Therefore, the procedure may not be applicable for your material if you are showing mono or few-layers materials.