Dude. It's not a secret. That's basically his recommended tab.
That is one way to play. That is what the most recent buff would suggest. Also, a look at Garen's recommended tab. In many matches, a build such as what you suggested could work, at least as a final build. (But why did you include the Suncape? That's a big armour item.)
Garen would normally build Deadman's Plate, and even rush it, because that item is considered "offensive" on Garen. Ninja Tabis are usually too good to pass up for what they offer, at that price. Fewer matches will require Garen to take Mercury Treads, for the extra Tenacity.
Or did you actually mean to include Warmog's, rather than Sunfire?
The extra defense in his most recent buff, theoretically gives Garen license to build more AD. Alternately, Garen is the kind of champion that uses tankiness to deal more damage. He scales with health more than he scales with AD.
The Black Cleaver/Trinity Force, Deadman's Plate, Sterak's Gage are core items and in most matches those will be the first three built, although not necessarily in that order.
Again, Deadman's Plate is core. It's not really optional, and it's not really about the armour. In the early game, Garen's full combo wearing only DMP is only about %10 less damage than if he had BC instead. Some high MMR Garen will rush DMP even against a full magic damage enemy.
Ideally, Garen tries to get HP on every item. (Ironically, Garen base stats are rather anemic for a top laner... his HP lvl 18 is just over 2k, about %8 less than others in his category.) A few exceptional items are frequently build that offer no health: Guardian Angel, Lord Dominik's Regards/Mortal Reminder.
Death's Dance is good on Garen, it's on his recommended tab, but difficult to fit into early build. And it has no HP of its own.
The other defensive items are situational - Randuin's Omen, and Spirit Visage (although many appreciate the boost that SV gives to Garen passive enough that they may try to find any excuse to build it.)
If you include Titanic Hydra, then you should probably also include Frozen Mallet as an alternative.
Once you realize that you are committed to building Sterak's Gage, then you really want get as much HP as possible (for the shield). Items like Youmuu's, ZZ'rot Portal, even Infinity Edge become less attractive.
A look at riste's recent matches shows that he was playing around with Lethality items and ZZ'rot recently, to good advantage, but it seems that he was already ahead when he decided to use these.
Almost as much as Garen scales with HP, if not more so, Garen scales with MS. Movement Speed is a precious statistic in League in general, but Garen craves it more than most others. And, indeed, most of his standard items, and most of the appealing nonstandard items, come with some MS attached.
Garen's scalings are largely irrelevant of any stat that he can purchase, illustrated by this extreme example: Garen is in a relatively good spot (for him) right now, but this hasn't always been the case over the past two years. At times, "tank Garen" seemed to be the most viable build. It was discovered that building tank magic damage items and magic pen, along with a rune like Grasp or old Conqueror (or even Phase Rush or Electrocute) would yield considerable damage. Sorc Shoes, Sunfire Cape, Thornmail and so on with Abyssal Mask for MR - MarineRevenge's "Weedzard Garen". (He did normally fit a Black Cleaver in there somewhere.)
I'll point out that your build, as you have chosen it, contains %50 CDR. If one is committed to it, and takes Transcendence and the CDR shard, then a yield of at least 36AD.