Crypto is not pure gambling.
Traders know how to read charts for day trading for example as there are certain patterns that happen before it goes up/down and so forth.
If you know how to analyze coins, you can predict (& have a high success rate) of when / how much it will grow, just like stocks. Join Crypto groups & check them out, certain players in the game know of it very well and have made people millions, literally.
When to sell would vary on the coin & your belief. Do you believe in this coin long-term? Keep it long-term (for ex: Ethereum). If it's a quick pump n dump, don't be greedy, better safe than sorry. You can also read charts & history gains/drops. I'm not an expert in reading them at all, I invest for long-term & I do some day-trading here and there based on what other reputable people suggest (& show charts & explanations for). Smart thing to do is to take principal out sometimes.