The USB Power Delivery support that comes with the Apple iPad Pro allows it to be charged faster with the Apple 29W USB-C Power Adapter. The Apple iPad Pro can also be charged by non-Apple USB Power Delivery based chargers. You will need to connect the USB Power Delivery charger with the USB-C to Lightning Cable.
Let's see how the Apple iPad Pro charges with the Google 18W USB-C Power Adapter using Granite River Labs USB Power Delivery test software.
The software initially shows the Google 18W USB-C Power Adapter offering 15 watts (5 volts @ 3 amps) and 18 watts (9 volts @ 2 amps) power profiles.
The Apple iPad Pro then requests 12 watts (5 volts @ 2.4 amps) of charging power from the Google 18W USB-C Power Adapter.
From the oscilloscope, the charger has established charging of 4.5 volts @ 1 amp with the Apple iPad Pro when fully charged.
While the Apple iPad Pro can get the fastest charging from its own Apple 29W USB-C Power Adapter, it can still be charged by other third party chargers like the Google 18W USB-C Power Adapter.