I would consider them to be gimmicks.
Gameplay to me is how you play the game and how the game inreracted to your control input method. In OOT for example, the giimmics as I said above is the two time zones and the ocarina. The gameplay mechanics are the Z targeting with Navi, the ability to...
My list of gimmics for each Zelda game would be as follows
Zelda 1 - Overworld/Underworld, 1st/2nd Quest
Zelda 2 - Spells, XP, the most open overworld of any Zelda game to date (in my opinion). Can't really say 2D platforming as that was a directional change for the series that in the long...
The best part of Zelda SNES was the music. It was amazinng and the best of any Zelda game to date. The rest of the game is alright. Nothing outstanding, and not the hardest LoZ game by a long shot but it's still fun to play from time to time.