A: If you suspect it is then it is. But, there is more to the answer and the question than that.
Our Lord used the casting of lots in selecting men to serve Him or to determine who is in the wrong --
Acts 1:23-26, Sam 14:41-42 -- and I mention this as some are familiar with these verses and question the use of dice. These two events, however, does not envolve gambling but in selection and service.
So, is that gambling? Not at all. Gambling is taking a chance on something usually based on "greed" and this makes it a type of idol worship. Gambling feeds on the ego and desire of wealth or the feeding of a euphoric feeling of excitement that when out of control can destroy one's own personal life or the family. A man's or woman's first obligation is God, then the family and Gambling away one's pay check or inheritance goes against this responsibility. The lust for fortune not earned or worked for is the Lust of Greed, an other kind of idol worship. We are not talking about the small bits of challenges we dare each other to at times. Simple challenges could be construed as gambling but what I mean by "gambling" which I believe is the question you are asking, is based not on this kind of friendly action among friends like playing cards or even rolling dice in a game, not based on making money but on friendly challenges among friends, a kind of competition, not a monetary lust. Hope this helps in understanding this -- Some in the Christian community have so little faith in our Lord's ability to deliver that they make up rules and laws, just like the Jews, as a way to keep them from sin.
In our New Covenant with our Creator sin can be anything that goes against your conscience -- if you feel uncomfortable with something, you are not as ease with something and you do it any way, then, it is sin. Perhaps the thing troubling you is not a real sin in the eyes of our God but you are not sure and it trouble you and you feel uneasy about it but you do it anyway, then it is sin. If, however, you are convinced it is not a sin and you do it, then it is not a sin. You Spirit mind, as a Child of God will guide you. For one thing, the Spirit of God will not guide you to violate any of His Commands. His Commands are clear, as expressed in the Ten Commandments, as they are called. Things outside of these Ten basic commandments are determined by our own feelings, those we are led to and when in doubt then avoid the thing troubling you. I know, that is too simple, right? It is man that makes following our Lord hard. It is men that want to bind you to them that trouble you with such things. HOWEVER -- listen closely -- for the benefit of new converts do not make a big thing out of things you know are not sin but troubles them, even to the point of avoiding anything that would trouble the new convert before they see the full truth as you do.