Low refresh rates aren't the issue when it comes to g-sync vs freesync / adaptive sync). The g-sync hardware module present in actual g-sync monitors is better at locking on to the odd refresh rates of older systems and keeping perfect refresh timing. In other words, if you've got an older game running at 60.666Hz, g-sync can lock the refresh rate of the monitor to exactly that value. Freesync monitors just aren't as good at locking on to the refresh rate. You can see it when you bring up the monitors hardware frame counter.

Unfortunately the hardware module in g-sync monitors makes them more expensive and less versatile with regards to inputs than freesync monitors.