Circle, Circular, Round
What's something with nine sides? Just add a drop and you'll have something that's almost a circle.
It's not quite a circle though. You just need to add a drop of liquid to the edges to smooth it out to become a full circle.
がん
まる
None
Do you know about the adorable cat Maru? If not, you should check it out. May seem like a distraction, but if you come back knowing the reading for this kanji, then it's totally worth while. When you think about a circle, think about Maru all rolled up in a little circle, being super adorable. Don't tell me it doesn't melt your heart just thinking about it. Alternatively, you could also look at this kanji and think of Maru inside of a box, with his little tail sticking out of it. Whatever works best for you, just get yourself to "Maru."
Obviously if you "don't know this reading" well enough, you should watch some more Maru videos. I'll understand... just make sure you come back someday, okay?