Is it possible to turn down the frequency on a LG 5K Ultrafine and will it help reduce GPU heat?
It's not possible to change the frequency and the physical resolution of this display.

IMHO changing that will not lower the thermal load of your MBP, because once you connect an external display the "external" GPU will be used and it has at least 10W of consumption. This is very apparent in the newer 16" MBP, where I measured ~20W of base load with no matter how many or what kind of monitors are connected.

edit: You can change the resolution (by option-clicking the Scaled label) or control-clicking Display preferences to show additional resolutions), but (at least on my setup) it barely makes any difference.