The concept of flakking is not clear to a beginner, and because gardeners are so integral to the maintenance of land, I can see why beginners would be confused initially that sending gardeners out on attacks would be a good thing, not a bad thing - "A weak vulnerable unit that I need to deal with my land? I'm not sending them on a suicide!"
However they are flak, they work as flak, they're balanced as flak, and the game is designed around them to be flak.
This leaves two solutions;
* An extra unit given to all players as a startup unit, called "Flak" - same stats as a gardener, same cost, everything the same, and then people have two units that both work as flak and can help with "flak layers" - causing a balancing issue. It also assumes that a newbie would know that the word "Flak" means "Cheap unit sent en-masse to absorb the blows and save the more important units", which they may not. It's also yet another unit for a first-timer to see. This 'solution' may solve the original problem while introducing several more - balancing issue problems being a major one.
* Find somewhere that beginners simply will not and can not miss, that will tell them "Hey dude, send gardeners on attacks when stealing land. Trust us, it's how you win! For full details on using gardeners to 'flak' your wheelies and geos, please read the manual here....". This solution seems to be simple, obvious, and the right thing to do. Just... where to put it and how to word it remains.
Someone else suggested putting it as a step in the tutorial, that and maybe a change in the description of gardeners.