No; it's only free on very full flights ("Als de vlucht erg vol is") when they have to take carry-on luggage from the late boarders. They will always make an announcement before boarding in such cases requesting people to volunteer to check in their bag for free in such scenarios.

In the absence of any such airline initiative, though, you can't just decide voluntarily to gate-check a bag for free; either they take a bag from you and charge you to check it because it's above your allowance/the allowed dimensions/weight, or they take your bag to be checked when they have run out of onboard space.