Some traders argue that investing in cryptocurrencies is gambling, but buying Ethereum isn't considered gambling in a legal sense. Any currency has fluctuations in price from day to day and over time. Ethereum's fluctuations aren't nearly as wild as Bitcoin's have been over the past year.
Ethereum has fluctuations, but it's considered one of the most stable cryptos at the moment. If you're using Ethereum for online casino gaming, you might consider those changes an additional gamble. The price of Ether has gone up over time, so it's a wager that's generally paid off for Ethereum users.