Resistant is a physical advantage, described in Basic Set, p. 80, and referenced in GURPS Powers™, pp. 71, 108, 167 & 169. There is an FAQ entry regarding Immunity to Metabolic Hazards and Temperature Tolerance here.
Immune or Immunity is the highest level that the Resistant advantage can be taken at, and so the Resistant advantage is often referred to as Immunity. For example, the Immunity to Metabolic Hazards advantage (above) is actually described under the Resistant entry in Basic Set.
GURPS Powers™, p. 71, discusses the relation between Resistant and power sources, while on p. 108, Resistant is mentioned as an advantage that can take the Force Field enhancement. On p. 167, it is mentioned in the Defending with Powers section, and in the Resisting Abilities section on p. 169.
- On resistance to "no resistance roll" effects, such as cyanide 
- On the difference between an attack not affecting something, and an immune defender 
- On how Resistance to All Influence Skills is equivalent to Indomitable for 15pts, allowing +3/+8 levels to be determined