Gameplay is a fuzzy term which describes the length, breadth and depth of the game experience. It is thus both qualitative and quantitative at the same time, but rarely specific. Game journalists often use it in a similar sense to the way that music journalists might use 'vibe'.
Game makers usually use as a catch-all term to describe whether the game's dynamic is satisfying. This can mean overall play time, difficulty, depth, dimensionality, ease of mastery, robustness, readability, opportunities for strategic or emergent play. It can also mean just the general quality of physical interaction (such as steering in a racing game), whether the game seems to sustain fun and so on.
Gameplay has recently fallen out of fashion in the business side of the industry in favour of metrics like engagement or retention, but is still widely used by players, reviewers and developers.