That's exactly my problem. I could fit it in both, but I've stuck it in mechanics for the moment because each class is special in its own way with abilities. Although I'm uncertain if this is correct and I'm making a portfolio to get into design at college.
"Gameplay" generally refers to the overall experience of playing (i.e. how the mechanics come together as a whole), where as "mechanics" usually refers to the actual inner workings of the game, i.e. the individual systems working together to create the gameplay experience.
"Classes", like in an RPG, are generally part of the status mechanics used for determining which abilities are allotted, how stats grow, etc. so if you absolutely had to tag them as either "gameplay" or "mechanics" they'd be mechanics.
But, complementing what you said, if the choice of class also interferes on how the game is played. For instance, choosing a Tank class plays very differently than choosing the Ranger class. That would be classified as gameplay.
@op
You really have to pick one? The definitive answer would require to understand the nature of the game we're talking about. A classic RPG where most of the interactions lie on the difference between stats and dice rolls, rather than the actual skill of the player, would make the answer really weight towards Mechanic.