Are Gateport landmarks considered path hexes for the purposes of connection to possible marks?

No. The Riftwalk Talent allows Units to move onto it as though it were a Path Hex, but it is not actually a Path Hex. All possible marks must always be connected by Path Terrain, so any adjustment to the play area or relocation of a possible Mark that would require the use of a Riftwalk to access the Mark is not allowed.