cheating in games is only venialy sinful, and i’m pretty sure even cheating on school work is venialy sinful…although i’ve heard different opinions on that, but i’ve only ever read that it’s venial, whereas i’ve only heard by word of mouth that it would ever be mortal.
but yeah, to answer your question, cheating in games without any money involved is venial…of course we still must take care not to commit any venial sins with as much diligence as we do with mortal sins, because remember that God is not so displeased with us when we fall from weakness as He is when we fall on purpose, whether mortal or venial…we seem to worry, more often than not, only about not offending God to much, rather than worrying about what we can do to please God, and strive not to offend Him at all.
anyways, hope this helps, take care.