Past participle verbs postmodifying nouns are non-finite clauses, not adjectives, not predicatives. So in your first example "gained" is a past-participial clause postmodifying "experience". Past-participial modifiers are 'bare' passives as evident from the admissibility of a by phrase in complement function. Semantically they are similar to relative clauses, cf. "The experience which was gained ..." The same applies to "used"; as a postmodifier it's a verb heading a past-participial clause modifying "materials". As an adjective, "used" is unusual in having a secondary meaning, second-hand.