I accidently tried this once after an argue with the ordinary support. I tried this some games and it worked shockingly good, I lost only two games since I play this.
Garen isn't only a sustain beast, but he can also deal incredible damage even without an offensive build. He is also very mobile and can finish an enemy if it is really needed. Although you don't have a heal, speed boost or anything else making a Support viable, you have a Speed Steroid, a Silence, which is pritty underrated since it even blockes flash, a big mammy shield, crazy damage, heavy health regen and a finishing Ult. This all may be very useful playing against squichy ADCs and Supports.

The most you can do as Support Garen is warding, farming, damaging and protecting. You should always be warding as needed in case they have a jungler, what is the standard case. Also, try to damage the Minions but only kill them if you have a loading on your relic shield.

But your main Quest and what makes the support Garen fun to play is damaging the enemys, you wait in their bush and silence the support, then you spin at them till they run, mostly they will only be on half life and you nearby full health. If your ADC helps he might even get an easy kill everytime you do this.

You take your Standard Top-Lane Runes and Masteries since you need both the sustain and damage.

Since most you can do is farming and damaging you build very Tanky and for Wards.

Garen has no heal and only one CC, these may cure this problem.

The build is not completed, but soon i will end it, till then I wish you much fun as Support Garen :)