The name seems to have two meanings, both having to do with protecting enclosures.
A gardener wasn't and isn't really someone who did flowerbeds or anything. It was a landscaper, just like today. A gardener was a specialist at his trade, he built ditches during war, cut vines on castles, and worked for nobility and lords.
It is driving me crazy trying to figure out the meaning. The first to bear the surname was probably our ancestor, Sir Osborne GARDNER, the Knight who invaded during the Norman Conquest. I believe the name was spelled GARDYNYR.
I'd love to speak to a linguist who really knows the variety of things a 'gardinier' could have been.
Your right about the different spellings.
I've kind of made my mind up that the name comes from the French word, 'garder', which means to protect.
I've read that Sir Osborne GARDNER saved the life of the King by killing an assassin. Gardner must have been a bodyguard for the King. Another interesting thing is that Gardner was about 60 years old when he was fighting in the Crusades.
My email address is firstname.lastname@example.org
and I have plenty of information on my Gardner family. You could tell me more about yours if you like. I've just placed mine together.