Are you talking about gate checking a regular piece of luggage or a special item like a stroller? Strollers or car seats can be gate checked at no charge.
Are you asking about gate checking your carry-on luggage or your stroller?
Basically carry-on luggage needs to fit either under the seat in front of you (smaller items such as purses or diaper bags), or in the overhead bin (larger items such as roller boards or backpacks). If you are one of the last few people on the plane and there happens to be no room left for your carry-on luggage in the overhead bins, the FAs will gate check your luggage at no cost to you. There is also no charge for gate checking your stroller.