How come you cannot determine the capacitance of a MOSFET gate based on the other supplied values on the datasheet?
You're being confounded by Cgd (same as Crss). Two main points: 


Cgd is very dependent on Vds.
Cgd gets swept through Vds-peak + Vgs-peak. Not just Vgs.


First, look at Figure 11 and Cdg. Note that at Vds = 25V, Cgd is the tabulated 220pF. But what is it at 5V ... 700pF. And what is it at 1V ... ~8000pF. 18nF not looking so unreasonable now. 

So, to get the 185nC number in the datasheet, as shown in Figure 10, you would have to integrate Cdg from 125V to 0V + Vgs-peak, and then add the Cgs contribution. 

Also relevant, 66660 and 74867.