It's a technicality and CYA statement on the part of Garmin, I suspect.
If an RNAV overlay is published, you can use GNSS equipment EXCLUSIVELY to fly the procedure.
If there is no overlay, the appropriate NavAids (VOR, DME, etc) MUST be tuned and their "raw data" displayed on the appropriate, certified instruments (per EASA, FAA, and all other A's I know of). Once that requirement is satisfied, NOTHING prevents you for actually using the GNSS equipment as your real-time reference while flying the procedure. Just remember that the tuned NavAids are primary and required, and that you cannot continue the procedure using the GNSS equipment if a primary NavAid should fail.