Diagnosis
That's a leaking pump seal. All pressure washer pumps have internal seals that keep the water in the high-pressure chamber separate from the oil bath that lubricates the pistons. When that seal wears out or fails, water drips out of the weep holes on the bottom of the pump housing.
How to Fix It
For the GPW1501, this isn't a user-serviceable repair in the traditional sense. The pump is a sealed unit. Your options are to 1) contact Greenworks for a replacement pump assembly under warranty if you're within the period, or 2) live with the drip if it's minor, as it won't immediately stop the machine from working. To minimize it, always run the washer for a minute after you're done cleaning to purge water from the pump, and store it in a warm, dry place.
If That Didn't Work
Make sure the leak is actually coming from the pump body and not from a loose hose connection or the quick-connect fitting. Dry everything off, then watch closely without running the machine to find the exact origin of the first drip.
Heads Up
This is the most common failure point on any pressure washer. It's often caused by letting the pump run dry (even for a short time) or not winterizing it if you live in a freezing climate. That's what kills the seals.