It depends on the model of the LG 5K. The original model is Thunderbolt only and does NOT work with the iPad Pro. The later revision added USB-C support.
I'm stuck in the same boat. My only external displays are 2 of the first gen LG UltraFine 5Ks. I'll have to wait for my DTG to arrive and see if it'll work (unless documentation gets updated somewhere) or find another monitor to use while working on that machine.