Several years ago, I somehow ran into these little figures, probably searching Ebay for Lego, and I pounced on them. They are the size of Lego Minifigures, but closer in design to being Kubrick ripoffs, if you have ever seen any of those.
There is precious little information about these online. I did find out from my research that these were sold individually in Gashapon machines, because of course they were; it just makes sense. Then apparently Yujin collected them as a box set, which is still very similar to the way candy toys are often packaged in Japan (mine was sealed and came with no candy).
The only site I could find that gave them a "review," so to speak, was extremely negative, saying they were cheap and not worth it. I disagree, primarily because Gatchaman/Battle of the Planets toys of any significance are few and far between. There is stuff out there, but precious little that's honestly done very well. I got all excited when Diamond Toys announced Battle of the Planets action figures in 2002, but they really weren't awesome (or very articulated), and I didn't end up buying them. Then along came a minifigure-sized set that included the whole gang, with accessories, for a reasonable price (at the time of course)! Now, admittedly, the engineering isn't Lego quality, and neither is the plastic. I don't think they would stand up to very active play, but I've kept them on constant display since I first got them.
I couldn't find any of these box sets currently on Ebay, so I'm not sure if they have gotten rare, or stupidly-expensive, over the years. I'd recommend them as a way to have the whole team (whether you call them Gatchaman or G-Force) in one, conveniently-sized place.