typical, what is to be expected, unexciting, hum-drum.
Probably derives from that fact that plants and odds and ends you can purchase for the purpose of putting in your garden are rather mundane and ordinary, in comparsion with their wild counterparts..
A RadioShack brand PC that you buy is an example of a garden variety computer. An SGI Octane is not garden variety.
The Game of Life is a garden variety board game. Axis and Allies is not a garden variety board game.