Coding itself really isn't all that hard. The hard part is really understanding the thing you want to code. Some people might not find it hard to code, but it would still take a substantial amount of effort to learn Overpower and map out all the possibilities and procedures that need to be handled...
As for Game-Maker, is that company still around? I've got an old (3.x I think) Game-Maker CD around here somewhere... unless they've seriously revamped it, my fuzzy memory of it suggests that you'd have a bit of difficulty doing a card game with it...