The rules governing approaches for GPS RNAV systems and WAAS-capable RNAV systems are different so it is important to be able to decipher the jargon when reading the AIM. The first section quoted below refers to GPS systems i.e. non-WAAS. The point of the special rules using GPS as an alternate has to do with the fact that non-WAAS GPS is supplementary navigation. So you can file with the destination or the alternate having only a GPS approach, but not both. WAAS-equipped aircraft can file for destinations and alternates having only GPS approaches. In both cases, the weather at the time of arrival must satisfy the LNAV or circling minimums or higher if noted.
AIM 1−1−17. Global Positioning System (GPS)
5. GPS Instrument Approach Procedures
(c) For flight planning purposes, TSO-C129() and TSO-C196()−equipped users (GPS users) whose navigation systems have fault detection and exclusion (FDE) capability, who perform a preflight RAIM prediction for the approach integrity at the airport where the RNAV (GPS) approach will be flown, and have proper knowledge and any required training and/or approval to conduct a GPS-based IAP, may file based on a GPS−based IAP at either the destination or the alternate airport, but not at both locations. At the alternate airport, pilots may plan for:
(1) Lateral navigation (LNAV) or circling minimum descent altitude (MDA);
(2) LNAV/vertical navigation (LNAV/ VNAV) DA, if equipped with and using approved barometric vertical navigation (baro-VNAV) equipment;
(3) RNP 0.3 DA on an RNAV (RNP) IAP, if they are specifically authorized users using approved baro-VNAV equipment and the pilot has verified required navigation performance (RNP) availability through an approved prediction program.
AIM 1−1−18. Wide Area Augmentation System (WAAS)
9 (a) Pilots with WAAS receivers may flight plan to use any instrument approach procedure authorized for use with their WAAS avionics as the planned approach at a required alternate, with the following restrictions. When using WAAS at an alternate airport, flight planning must be based on flying the RNAV (GPS) LNAV or circling minima line, or minima on a GPS approach procedure, or conventional approach procedure with “or GPS” in the title.
For more details, see Notice N 8900.218.