Well, with the disclaimer that I’m not a lawyer, and rental law varies by state and country….
Generally, if their apartment is unsanitary or a fire hazard, then it’s probable that you have cause to evict them. If they’re otherwise good tenants who pay the rent on time, you may want to threaten them with eviction and hope they clean up.
If there’s no unsanitary conditions or fire hazard, just a lot of clutter (which could be considered a fire hazard), then it depends on the lease. “Gross” sounds like a very subjective term, though we all know what you mean. So you have to compare the condition of the apartment with the definition’s of “clean” that you specified in the lease.
Also, most likely you specified various penalties for stained carpets, dirty walls, dirty appliances, broken windows, etc. so that if and when they move out, you can apply their deposit to cleaning the apartment.