To add to this: "level design" typically covers things like the design of paths through the level (both physically and plot/objectives) and visibility of paths affecting player thinking and choices (ie making it clear to the player how to progress, not get lost). These are "big scale" things, not fine detail.

"Gameplay design" typically covers things like movement, interaction and item/skill progression mechanics. The are "small scale" (or for inventories & skill trees: "no physical scale") things.

In practice the two terms do often overlap quite a bit, so you can argue basically anything to be in either category.