Tcvennen, an OSAP is not an FMS approach; the FMS
MAY be used to set up a DFAP and FAF, however the radar remains the primary sensor to identify the destination and the 1/2 mile separation required by the FAA in the relevant circular (the specific number escapes my memory).
The alternate limitation also is in regard to the use of GPS as primary sensor for the approach.
Notice that the alternate minimums are generally labeled NA in those IAPs to airports where the only available approach is GPS.
Unless you have a published GPS approach to a specific offshore block lease in the GOM.
HTH