While both prenominal and postnominal relative clauses may be formed by gapping, gapping is the overwhelmingly dominant mode of relative clause formation in prenominals.
Standardized
IF relative clauses are prenominal, THEN relativization is primarily by gapping.
Cf. Downing’s statement: “With few exceptions in prenominal relative clauses, the relative NP is obligatorily deleted in at least some positions.”