Gundams are very different than other models. They come molded in color, so some don't paint them. They are very well engineered kits, so painting can actually be a big challenge. Since the parts fit so tightly, you often have to sand the parts so that once painted, they can still move.
The also come in various grades.
RG: they are 1/144 scale, very well engineered, and articulated. They are also not very expensive.
HG: Kind of the lower line. Scale varies, and the details are soft, and the mobile suits are not very articulate.
MG: The deluxe model. Well detailed, articulated, usually 1/100 scale.
PG: This is the top of the line, expensive, very well detailed, indivual fingers will move on the hands. Expect to pay $200 minimum for one. 1/60 scale.
Mega sized. 1/48 scale, not the best detail, just very large.
You can find out more information here:
http://www.scalemodeladdict.com/SMF/index.php/topic,6938.0.html