As for pure GPS SIDs i would agree, however for P-RNAV SIDs or RNAV/GPS SIDs only the applicable RNAV ANP is required, not a GPS if you can achieve the ANP without it. We had all our 737 classics without GPS certified to P-RNAV and could fly everything down to RNP 0,5 legally with them, except for pure GPS-Procedures.