@Franki What ever happened to you and your ever-present gardeners?
I had the same issue with massage therapists last year but, after 2 weeks, they (yes, they multiplied) went away after I moved the massage tables to family inventory.
Today, I've noticed that the gardener I hired yesterday "spent the night" standing in my sims' garden. Dismissing or firing him doesn't make him leave.
After traveling with the gardener to a different lot then returning home, my sim found the gardener there already.
I'm limited to what I can do as a solution or work-around, because I'm playing this particular save as one of the forum's dynasty challenges.
Did your gardener issue persist until you created a new household?
I just checked EA Answers and I see that the bug thread for this issue was revived a week ago.Grrrr