SBAS
From my notes, I think it's an issue of SBAS. App mode needs SBAS to function and the aircraft uses NAV instead if reliant upon RAIM instead. You should be under the GAGAN SBAS at that airfield. You should check its enabled on your GTN750 and if so, check you have 3D DIFF NAV in the GPS Status menu.
It's still perfectly valid to fly the approach using NAV, being reliant on RAIM for integrity. The scaling of the RNP is Garmin thing all approaches it generates are angular- of course strictly for an LNAV it should be a fixed lateral width all the way in. See the excellent PBN Manual produced by the PPL/IR group for more.