The Babbo website has a useful comparison of green garlic and ramps, another up and coming alternative to leeks and green onions. Here's the chunk on garlic:
Green garlic is actually white garlic that has been picked while the long stalks are still tender and green (adolescent, if you will), and before the bulb has had a chance to fully develop into common garlic. Much more mild than mature garlic (one stalk of green garlic is equivalent in pungency to about 3 cloves mature garlic), green garlic stalks can be used much like any other greens.I consistently read that green garlic is more mild than the full-grown adult version, and this may be so, but I find that it is somehow more redolent.
Doug Stonebreaker, who works the Prather Ranch booth at the Grand Lake Farmer's Market, said he picked some up last week to marinate lamb steaks in. At Dopo they use it in potato soup and on pizzas. There are several recipes for green garlic located at the bottom of this page on Gourmet Sleuth dot com.