High data rate communications have become possible due to improvements in microelectronics, and is based on new topological concepts of architecture. The development of 5G networks using the Ka-band is one of the challenges for new telecommunication systems based on individually controlled active antennas (AESE). The control system is usually developed using silicon processes but this approach does not allow a fully embedded transceiver as the front-end module must be connected to the core-chip module and to its digital control interface. With the GaN power technologies providing high PAE, an interesting issue consists in designing a fully integrated GaN MMIC multi-function chip for highly integrated systems. This work presents, for the first time, a GaN attenuator-phase shifter covering a wide range of frequencies from 30 GHz to 40 GHz. Moreover, this circuit is a balanced version of the attenuator-phase shifter to target an improved linearity and to take full advantage of GaN power amplifiers.