Thanks dude! Im glad it helped you, i still have some stuff to add, but overall this is all it has to offer.
About your question, Ghost + Flash with PR are not really bad, though i don't agree on using them everygame, but it sure can go work, and makes Garen unkiteable once you get on top of someone, though that works really well because it's high elo, but genreally them damage from conqeuror+ ignite is really valuable for Garen. And he gets a ton of speed from his items any way, so i personally think it comes to preference.
PR allows Garen to escape from sticky situations as well to be stick on to an enemy while having almost slow inmunity. So i can see why he would run PR every game in High elo.
In MY opinion too much movement speed in Everygame it's a little overkill, because you also need damage to back up all your stickness and chasing power, although Ghost and PR scale super well once you get your core items. Hope this helps.

Since you laready got a response im gonna give my take on the matchup, reglardless.
In short Riven generally wins extended fights, so you should try to silence her with Q and deal some Damage with E and then back off, so she can't respond and you are unscratched, and as Ulises said, when she is about to use her full combo on you just W, which will also reduce her W stun duration. Post-6 if Riven has some ability on cooldown try to quickly punish her and go agressive, and if she underestimates you ignite and Ult should kill her, i think garen's R, as an execute, is far more powerful than riven's, so if you don't get harrassed to much you shoiuld win fights agaisnt her early on, later on it's really hard to deal with her constant dashes and shields, your bets bet is bulding mortal reminder and going with as much damage as possible to quickly execute her before she get's away.

Riven is a pretty easy Matchup for Garen. Just W her combo.She cant do much to you unless she gets fed

Thanks!