Since the GoPro HERO5 Black supports charging through USB Power Delivery over USB Type-C, we'll see if it can be charged using the third party Anker PowerCore+ 26800mAh PD with 27W PD Portable Charger Bundle which is a USB Power Delivery based portable charger.
Using the Anker 30 watts based portable battery portion, we direct connect it to the GoPro HERO5 Black with a USB Type-C to Type-C cable. The USB Power Delivery negotiations show Anker offering 15 watts (5 volts @ 3 amps) and 30 watts (15 volts @ 2 amps) to the GoPro at 6% battery.
The GoPro HERO5 Black then requests 8.25 watts (5 volts @ 1.65 amps) which is then provided by the Anker battery charger.
Later we can see Anker declares 15 watts (5 volts @ 3 amps), 27 watts (9 volts @ 3 amps), 30 watts (12 volts @ 2.5 amps), 30 watts (15 volts @ 2 amps), and 25 watts (20 volts @ 1.25 amps). Oddly Anker doesn't claim to support 30 watts (12 volts @ 2.5 amps) on its casing.
The GoPro HERO5 Black still insists on 8.25 watts (5 volts @ 1.65 amps) of charging which Anker also complies.
We can further confirm that the GoPro when on is actually charging at around 7.8 watts (5.2 volts @ 1.5 amps) after the first 10 seconds with the Anker battery charger. When first connected to the charger at 6% battery, the GoPro is seen to start pulling around 6.5 watts (5 volts @ 1.3 amps) but then draws a higher current of about 1.8 amps when at 5.2 volts for a brief period before settling on 7.8 watts (5.2 volts @ 1.5 amps) of charging. Notice both the 1.8 amps and 5.2 volts drawn don't seem to comply with the 1.65 amps and 5 volts as negotiated over USB Power Delivery.
We also observed the GoPro HERO5 Black pulling similar power from the USB Power Delivery based GoPro Supercharger in this earlier review.