Page "Gamma World" Paragraph 8

Characters in all versions of Gamma World earn experience points during their adventures, which cause the character's Rank ( in some editions, Level ) to increase.

Unlike D & D, however, the first two editions of Gamma World do not use a concept of character class, and increases in Rank do not affect the character's skills or combat abilities.

In fact, in the first three editions of the game, character rank is primarily a measure of the character's social prestige.