It's a tricky one, but fortunately Gnomes are awesome
Try starting with 1 gardener, and use him to take the city to the south. After the gardener just build wolfriders for a while - be aggressive with them and use them to kill the runners that are out taking cities. Target the runners on cities that are close to being captured first. Use the gardener to follow and heal the wolfriders. Build a second gardener if you only have $1600 to spend in a round.
You shouldn't need to worry about capturing cities too much - I had just 2 gnome cities for most of the game. As long as you are building up your army while keeping the comps city/army count down, then you win.