What we find gross is mostly arbitrary and emotional. It’s loosely based on the perception of filth but most people who find something gross will continue to find that thing gross even if they know it’s clean. If someone feels like snakes are gross, they watch you take a snake and scrub it clean with soap and water (don’t actually do this obviously) and you try to hand them the scrubbed snake, most people would continue to call it gross. Furthermore, if you ask most people why they find something gross, they won’t be able to give you a real answer. (Food seems to be an exception but we mean something entirely different and much more specific when calling food gross unless we are saying that the food is somehow foul or unclean)

In most cases, when someone calls something gross, they are doing so as a reaction to a feeling it gives them. Whatever they say after that tends to be some form of post-hoc justification to legitimize that feeling.