I don't think so that holding is a form of gambling. Gambling involves game wherein you can win or lose. Holding is just your strategy to make profit so with this you need to always check the market to monitor it's price not like in gambling. In holding once you can't sell your bitcoin during its high price only the profit will lose not like in gambling that if you lose the game you don't have profit and you lose also the capital so you need to go back with nothing. So gambling is worst than holding.
For me, gambling is anything that includes risk. Holding is also risky hence we can say that it is gambling. Let's say you are holding a crypto currency and you never know whether it will be profitable to in the future or not, so you hold it risking it. Investing in a crypto currency is always a risk since we are dealing with the most volatile investments in the world.