Should I mention video gaming in my college application?

Over the past 4 years I've been almost fully responsible with running the ins and outs of a guild of players in an online game.

It's been nearly a full time job to manage literally hundreds of people ranking from the new members all the way up the chain. We are one of the top guilds in the US and this has been through almost my own effort the entire time. I've interviewed many candidates personally before relegating this duty to another person.

Is this a suitable skill to include in my college application? If so, how could I use it? The experience of running this guild of players has given me valuable leadership insight that I feel is important to highlight.

Definitely! It sounds like this is a big part of your life that demonstrates your organizational skills and leadership potential. My first recommendation would be to write an essay centered around this experience and what you learned from it. This is a great subject for a college essay because it shows that you're a person with unique interests who is passionate and driven even outside of a controlled academic environment. If you don't end up writing your college essay about it, you should at least include it as an extracurricular activity or add a description of what you did in the additional information section. Anything that takes up this much of your time and demonstrates this level of dedication is exactly what you should be including in your application.