There are two boards for GBA TV out:
GBA Transverter - one I use, have one of them, used on my Nintendo 64 Advance project, ripped apart for the board and nearing completion of a GBA plug'n'play for my Mini project (will post in next day or two after painting). This can do RGB and composite. As hard to get hold of these days as an honest public-spirited politician.
GBA TV Converter - this can do RGB too; hard to get hold of but you can get them sometimes. Issue is that they clip to the back of the GBA LCD ribbon, which is rather hit and miss and a daft way to do it. The Transverter has a ribbon to connect to the GBA like the LCD screen does, so easy. Try and find one on e-bay.
It was hard to get this information a couple of years ago when I was searching for it.
RGB pinouts:
You also need to put in C-sync and also ground. In pic below, red=C-sync, blue=ground.
Look at this site for more details. It looks like the guy who make this RGB mod is the one responsible for the demise of the GBA Transverter for patent reasons, anyway, screenshots and pinout on his page!
http://www.konlabs.com/articles_data/gb ... /index.htm
GBA TV Converter:
Also worth looking at:
http://www.konlabs.com/articles_data/gb ... /index.htm