It's not out in the US on DVD. Manga has given a couple dates over the years, but I sincerely believe we'll probably never see it. At least from that company ayway. I'd pick up the Japanese release, but it's about $250 for the set.
Giant Robo is an incredible series. Characters, story, music, animation, and designs all work wonderfully together. I'm more than willing to forgive them for taking ten years to make the whole thing. If you haven't seen it yet, do yourelf a favor and watch it as soon as possible.