GDM is enabled by default for memory speeds greater than DDR4-2667 per the DDR4 spec. GDM allows the RAM to use a clock that’s one half the true DRAM frequency for the purposes of latching (storing a value) on the memory’s command or address buses. This conservative latching can potentially allow for higher clockspeeds, broader compatibility, and better stability.
For overclockers, Geardown Mode will be noteworthy because it also tells the memory subsystem to "disregard" the command rate set in the BIOS. As 1T command rates can be beneficial (though tough to maintain) for performance, the chart below is really asking whether it’s useful to run GDM if the desired memory clockspeed can be achieved...