The short answer: CodeChef.

The long answer: for any definition of "gamified" (not sure what you are looking for), it is unlikely that you will get better simply because of this "gamification." You need to practice smarter and more efficiently, and do not deceive yourself that changing the external environment (e.g. "gamification") will make you better when you in actuality need to change your internal environment (e.g. your approach to learning and practicing, mental model of topics and concepts).

However, if you insist, CF Visualizer is a way to "gamify" the CF problem set. The "game" here is to shift the histogram of your problem rating to higher ratings.

Indeed, your rating graph is a "gamification" too. The "game" is to do well in contests and increase rating without cheating.

That is all.

Try this: CodinGame