Actually People dont know the value of bitcoin or dont try to understand the bitcoin processing. Typically i think they say it as gambling because it is a decentralised digital currency and they fear to do transactions with untrusted or unregulated payment option. The way it fluctuates make people feel that trading bitcoin is one of gambling things. But they dont understand that it is an asset and your investing your money, time, knowledge and skills to get profit from it and so you cant say it as a gambling.
Yes, you can saying it is gambling, because it is.
Read: https://stockbet.com/#/support/gambling