GP2, the prototype installed at INFN laboratories in Pisa, is the seed device for the next generation heterolitic active- stabilized RLs. It has been designed in order to gain a long term stability and accuracy of the scale factor, via a precise control of the systematic errors related to the fluctuation of the cavity geometry and the active medium parameters. In particular, it is dedicated to implement a length stabilization of the diagonal cavities using optical interferometric techniques. The granite slab whereon the cavity is placed is oriented along the local latitude in order to maximize the Sagnac signal and minimize the orientation errors on scale factor. The four mirrors holders are placed at the corner of a square granite slab and the vacuum chamber encloses the beam optical path along a square loop 1.60 m length in side. The slab whereon the holders are mounted is made of precise black granite, a rock well suited for metrology application for his long term thermal and dimensional stability, high flatness accuracy, high bending strength and insensitivity to mechanical overloading. It has been machined with a precision better than 10 μm to guarantee a preliminary well positioning of the corner mirrors. The GP2 vacuum chamber has been designed in order to give access to the diagonal resonators by enclosing the path of two external laser beams along these, as well as the perimeter path of the counter- propagating beams. To stabilize the absolute length of a square RL diagonal resonators with respect to an interrogating high-stability laser we worked out an interferometric metrology technique. The technique, already tested last year on two Fabry-Perot resonators simulating on an optical bench the diagonals of a ring laser [Belfi, J. (2014). Interferometric length metrology for the dimensional control of ultra-stable Ring Laser Gyroscopes.], will be implemented on GP2 in the next months.