Build looks pretty solid.

Runes need a bit of work I think.
Almost nobody runs ArPen and AD Marks together (at least that I know of).
I would reccommend you say 9x whichever you believe to be stronger, and elaborate on other options in a "Runes" section.

I get that you probably haven't had *too* much time to test him out, and you were excited to publish your guide, but it needs a lot more work put into it.
It is too brief, and it could use some more BBCoding to make it look better.
Also, the purple letters in Reptar are quite difficult to see.

I hope you decide to accept some of the suggestions I have made in this comment, and I will check back soon to see if there have been imporvements (and maybe leave an upvote if it is significantly better!)

Good luck in your guide-writing journeys.

why not: 3 Attack Damage essences and 9 armor penetration marks?
better than some pieces..

Dude... get your glyphs together man.

Build is good, the only difference between yours and mine is I tend to get frozen mallet instead of warmogs (with the runes and trinity force you still end up with bonus 1000 health) for the extra AD (since tri-force and BotRK don't give very much,although BotRK passive and active make up for it) and the extra slow you need to keep people trapped mid-late game even in mini-gnar form, or to slow them down before they try to escape, making it easier to hit them with Mega-gnar's Q, followed by a hop or immediate W.