Generalized frequency division multiplexing (GFDM) has become one of the most important waveform candidates for air interface in 5G communications. However, like the standard orthogonal frequency division multiplexing (OFDM), its direct-conversion receivers are vulnerable to radio frequency impairments, due to their cost and size constraints. In this paper, the performance deterioration resulting from inphase/quadrature (I/Q) imbalance is first analyzed in terms of signal-to-interference plus noise ratio (SINR) for typical GFDM receivers. Next, a blind adaptive I/Q imbalance compensator based on normalized complex least mean square (NCLMS), originally designed for OFDM receivers, is extended for GFDM ones. In order to provide more physical insight into its compensation capability, a full second-order performance assessment is established, via a joint consideration of the weight error variance and its complementary variance of the proposed compensator in both the transient and steady-state stages. Apart from an accurate evaluation on its overall self-image attenuation performance, the proposed full second-order analysis is also able to quantify the individual contributions from both the I and Q channels of the compensator, an important finding missing in the literature and not possible to discover by using the standard variance analysis only. This analysis also facilitates theoretical quantifications of SINR improvements by NCLMS for GFDM receivers. Simulations on GFDM waveforms support the analysis.