It is myunderstanding that case law supports RKBA in your home and grounds, even when it is a rental property. I do not believe that the landlord can legally evict you for owning, carrying, or using a firearm legally.
However if the other tenant actually threatened someone with a firearm, then that is illegal, and he can be evicted for assaulting the gardener. Whether that happened or not is something even you probably don't know for sure. Regardless, you can carry on. Don't be mean to the gardener.
-- Sandy (WA)