Flexible routing of visual input signals has been linked to changing patterns of synchrony between gamma rhythms of distant neural groups. It has been proposed that attention increases distant gamma-band synchrony by increasing the gamma frequency of the sender group processing the attended stimulus. This stimulus is subsequently preferentially processed. However, the frequency of gamma activity fluctuates over time, varies between areas, and depends on stimulus characteristics, which may complicate entrainment between oscillators. Here, we investigate the relationship between the frequency of gamma-band activity and its inter-areal synchronization (measured as phase coherence (PC)) in the local field potentials (LFP) of two nearby V1-populations (senders) providing input to one V4-population (receiver). LFPs were recorded from a chronically implanted array in V1 and single electrodes in V4, while macaque monkeys performed an attention-demanding shape-tracking task. We report a higher gamma-band frequency in the attended V1 population compared to the non-attended V1 population. For the attended V1 population, we also find larger inter-areal PC with the V4 population than for the non-attended V1 population. Remarkably, inter-areal PC was maximal at the peak frequency of the attended V1 population, while the V4 frequency was higher than the V1 frequency on average. Moreover, the frequency variations in V4 during the trial significantly correlated with the frequency variations of the attended V1 population, but not with those of the non-attended V1 population. These frequency co-variations between areas indicate flexible, attention-dependent coupling between selected populations and might help synchrony to overcome the natural variability of gamma frequency.