When you say "tú ganas mucho dinero" in the present form "ganas", you usually refer to money that you earn. Whereas if you say "tu ganaste/ has ganado mucho dinero", in the past form, it is referred as money you won in a game. Specially when talking about money.
But this is not a rule! It's the only way I can try to explain the difference. Because when you are playing a game (eg. Monopoly) and someone is dominating the game, and you tell them "¡Tú siempre ganas!" you are telling them "you always win" not "earn". P.s. I'm a native speaker, that's how we use it :)
I'm no expert but I can only assume it depends on the context. As with, "Tú ganas mucho dinero." it would be less likely that you 'win' a lot of money (in somewhat of a daily occurrence) but probably more likely that you 'won' a lot of money. And so, knowing this, 'earn' would be the best fit.
I was going to ask if you would use "ganas mucho dinero" or a different verb to compliment a successful professional poker player. Your example is better because a professional poker player is both winning a game and earning a living. (We had a caregiver for my father-in-law who claimed to win a lot of money playing the Pennsylvania lottery. State lottery payout ratios are much lower than the payout ratios for most games of chance in a casino.)
You have.to watch for parts of speech. In your expression ganas is a plural noun.
But in Duo's sentence ganas is the second person present indicative of the verb Ghana's to win or earn. There are quite a few verbs which in some conjugation are the same as some other non related word. A couple of other examples are Como I eat & Como like/as. Or Vino he came (third person preterite) & vino wine.
The bottom line is that for the most part, if a sentence is referring to an action which is taking place in the present the common translation to English would be to the present progressive if the verb expressed an active action and not thinking or feeling or knowing, etc. So in languages on Duo which do not have a progressive tense like German or French both translations are accepted. The present tense in English with these verbs is used to convey habitual actions, sort of like the imperfect in Spanish but not in the past. But Spanish does have a progressive tense although it is used a lot less frequently and only to emphasize the continuous nature of the action.
So Duo has adopted a convention of translating tense for tense. This just allows them to be drilling what they want to be drilling. But occasionally there are sentences that just don't quite work in the present and require a translation into the progressive to sound right in English. That's when you get the I am doing construction. But it is important to recognize that although Duo uses this convention for its own convenience, many if not most times you see the present tense in conversation, the present progressive translation would be the correct one in English. Certainly any answer to the question Qué haces would be. Qué haces? What are you doing? Yo como I am eating. Yo trabajo, I am working. But in Spanish the equivalent estoy comiendo or estoy trabajando would only be used to emphasize that you are in the middle of the process.
Duo likes you to reflect the same tense used in the Spanish. It is its way of determining whether you are recognizing what it is. In context based translations the present tense in Spanish (and any other language I am familiar with) will be most often translated into English using the Present Progressive as this is what English uses to talk about what is happening now. Spanish uses it only as a sort of emphasis where we might say I am right in the middle of.... Or I am just...
Without any content you can t definitively say whether it is earn or win. Both should be accepted. But I think as a general statement like it seems to be here, earn would be more common. In the real world the context would tell you. But in general since most people earn more money in their lives than they win, earn is the more common meaning you will hear for ganar and therefore sort of the "default" meaning. If you were in Los Vegas, talking about the lottery or some similar situation than the context would clearly distinguish the topic as winnings, but in general earn is the more common meaning.
It could be just a Duo fluke, but it is probably because you wrote a lot as one word. Although you will see it that way, alot is not a word.