Hör auf zu spielen.

English Translation

Quit gambling.

Why is this “gambling” specifically? Doesn’t this just translate to “stop playing”, or is this an idiom?

So let’s start by sayjng that, I think that spielen is indeed better translated (especially without context) as to play. However, I’m guessing the translation went from english to german, instead of from german to english, and so the translator had to translate “to gamble”.
The wiktionary gives two translations: “wetten” and “spielen”. I believe “wetten”, as might be appreciated by the ressemblance of both words, fits better as a translation of “to bet” (and that is actually the first meaning when you check the entry ). So that leaves us with “spielen”.
As far as I know, a more complete translation would be “um Geld spielen”, but “um Geld” tends to not to be explicit, and is usually obvious from the context.

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