We like to go on weekend trips with our toddler on low-cost airlines. We often hike at the destination, and generally prefer child carriers and baby backpacks (like those listed here, i.e., a backpack that you wear that your child can sit in/on) to strollers, so we bring one of these as well as a car seat. Frontier Airlines has always allowed us to gate-check both without charge. They often do give us a somewhat confused look when we explain that the carrier is functionally equivalent to a stroller, and we keep our child in it at check-in to demonstrate that point as easily as possible.
So I know that this works - we've done this on 4-5 trips. But we're flying on Spirit Airlines for the first time in the near future, and I remain a bit concerned that they might not accept our argument that it's equivalent to a stroller (and thus should be eligible for gate-check with no charge). Has anybody ever tried to gate-check a child carrier / baby backpack (on an airline that allows stroller gate-checks without charge) and not been allowed to do so?