5th Generation (5G) and Industry 4.0 (I4.0) cellular systems have many different use-cases which impose a diverse set of requirements on a candidate Multi-Carrier System (MCS). These requirements demand high flexibility and reconfigurability from any candidate waveform. In this paper we present gr-gfdm a Software-Defined Radio (SDR) implementation of the waveform candidate Generalized Frequency Division Multiplexing (GFDM) in GNU Radio, discuss the details of our implementation and show how the SDR flexibility can be employed for use in simulations and field trials. With a careful design the same implementation can be used in testing, simulation, field trials and interfaces to other programming languages are easily added. Furthermore we facilitate an over-the-air transmission with our implementation in order to demonstrate its capabilities, where we could achieve sample rates of up to 25 MS/s.