The Bible does not mention the word ‘gambling’, although the soldiers who crucified Jesus were gambling, in a sense, when they cast lots for his garments. However, the most common motivation for people who gamble is greed, or covetousness, and this is forbidden in the Ten Commandments and Jesus Christ condemns it (Luke 12:15). In extreme cases, gambling becomes an obsession and can cause very great harm. Paul wrote
For the love of money is a root of all kinds of evils. It is through this craving that some have wandered away from the faith and pierced themselves with many pangs. (1 Timothy 6:10)
Even when gambling is under control, the gambler must be careful that someone else will not follow the example, resulting in destructive effects.
Additionally, gambling directly has a negative impact on the people you are gambling against: you take their money without any mutually beneficial transaction having occurred — you gain at their expense. This goes against the second of the two greatest laws, to ‘love your neighbor as yourself’ (Mark 12:31), and Jesus’ command that ‘whatever you wish that others would do to you, do also to them’ (Mat. 7:12). In this vein, the attitude of a gambler is also rather unChristian: a gambler wants their opponent to loose, and they use bluffs and deception to try to deceive their opponent.