Playing games while you have the charger plugged in and the battery is full is completely fine. Batteries naturally wear down over time, so playing games didn't cause that. You can't increase your battery health at this point, but if you want to keep your battery healthy for longer, here are some general suggestions if you're able to follow them:
- Try not to allow your battery to drain all the way down to 0%.
- If your system supports this and you can work this way, set your battery to only charge to 80% rather than 100%. Batteries don't like being kept at maximum charge all the time. However, doing this of course means that you'll only ever get to use 80% of your battery, so some people would rather have longer battery life rather than keeping their battery healthy for longer.
- Again if your system supports this, set the minimum charge to a lower value, like 50-60%. If you do this, then instead of the system keeping the battery at 100% [or the maximum value you specify], the battery will be allowed to naturally lose charge over several days before getting charged again. This again allows the battery to stay healthy for longer, but of course the risk is that you might disconnect your system from the charger at a time when the battery is down at 50-60% -- so again, you have to decide whether you would rather have longer battery health or extra convenience.