There are a few basic grades to Gundam kits, but many more special grades such as SD (non-scale Super Deformed), HGUC (High Grade Universal Century), and Reborn One Hundred. The basic and common ones are High Grade, Real Grade, Master Grade, and Perfect Grade. The grade mainly determines the scale but also indicates the number of parts and the difficulty of the kit. Grades also determine the level of detail and articulation, with High Grade kits (1:144 or 1:100) being the easiest to build but the most basic ones in terms of quality, leading up to Perfect Grade at 1:60 scale which are made of higher quality plastic and come with the occasional screw, metal part, or LED light. It is advised that beginners in Gunpla building go for High Grades and SD kits because these are easier to build. Master Grades and Perfect Grades are more challenging but rewarding for advanced modellers and builders. Depending on my mood, sometimes I prefer putting together a small kit like an SD because they're cheap and fun to build when I don't feel like working on a bigger project. All in all, every scale and grade has something nice to offer no matter your experience level.
Below is a chart comparing the sizes of different grades:
Below is a chart comparing the sizes of different grades: