Well for me it can't be considered as gambling because nothing is at stake if ever you lose, it's just a friendly bet, more on a challenge as you've said. I also tried this kind of challenge with my sister, instead of money we're betting on who's the one who will do the household chores through playing chess. Unfortunately I always lose because my sister is quite good in it, back on the days gadgets and having an internet are not yet popular. Anyway, it's been said many times here, when there's no money or valuable things are at stake, it's not gambling.
Yes because gambling has to do with bets and potential winning so anything outside that is no longer gambling, if you play simple and popular games with friends such as ludo, wots and draft games there all just traditional fun games and can not be categorized under gambling unless the players involved decide to stake money on the game that the only we can refer to it as gambling, so what makes gambling to be referred to as such is the staked amount that is out for the contest and possibly being won.
But for obvious reasons, some of the folks who are addicted to this kind on none betting game are higher than those who are actually addicted to real gambling where they have to stake an amount.
you don't need to invest money to play gambling, for example you can bet with your friend that on test X you will get a higher grade and that if you lose you will do everything your friend asks that is related to school activities and not put your life and physical integrity at risk, so this can be considered a gambling, and you and your friend did not put money to bet, so what is a gambling? in my opinion gambling is any game where the player has no control over the end result, I remember that when I was a kid I made a lot of bets with my friends
and we didn't have money to put on these bets, for example we organized soccer game tournaments and each team put food in the pot, for example bread, juice, fruit and other things and the winner got all the food, that was a gambling, but we didn't put money in because we were kids and we didn't have any money, we also played cards and bet with food, for example we bet on a piece of cake and the winner got all the pieces of cake, those were good times we had, unfortunately in today's world children are left with electronic devices and do not care about interaction in the real world