What is it? Only one of the most essential games for GBA, and very possibly one of the greatest and most expansive puzzle games for any system. (How's that for effusive)
The premise is deceptively simple -- you're given a grid with a number of blocks placed on it, a stock of your own blocks, and a cannon with which you can shoot out your stock, or suck them back in. You're limited to moving left and right at the bottom edge of your grid, but the entire grid itself can be rotated (guru) 90 degrees at any time. The object is to place all of your blocks into pre-determined positions around the grid by knocking them into other blocks, piling up, rotating, and then taking away walls for them to rest against, and in general building up and tearing down patterns to get blocks in places you're amazed you could get them to by simply shooting in a straight line. Once you've placed all of your stock, the grid peels away and reveals some sort of iconic picture of like a bear, or a sheriff's badge, or a pile of poop, etc.
Later courses add new challenges like spring blocks -- which shoot your fired block straight back into your cannon -- and free-moving blocks which advance one space in the grid when you hit them with your own.
There are hundreds and hundreds of puzzles available in the game, and at least a handful are guaranteed to make you tear your hair out by the roots. I don't know anyone that's 100% completed the game, and we've been playing for at least 2 years now. That's why I say 'expansive.'
Anyway, talk is cheap -- you can download a PC demo and try it out for yourself. Give it a go at
http://www.tcp-ip.or.jp/~minaduki/kikaku/kikaku/01/file/gururoji_mVer.zip and then do yourself a huge favor and track it down at all costs.