I've tanked with Garen a lot of times.
Don't do it.
Why?:
-His only CC/Debuff is a silence.
-His ultimate is aimed at finishing targets off, something a tank IS NOT supposed to do.
-Does not have a single support skill. (Yes plenty of tanks don't. But others also have more than just a single target silence)
Playing an AP carry as a tank would do a better job than Garen. For example Veigar simply because he can still injure their APs pretty badly and has an AoE Stun.
Okay and to settle these dumb arguments...
Me (Tank), Chex, +1 person
vs
Sony (Tank Garen), Sulfate (Teemo), +1 person
On Twisted Treeline because 3v3 wouldn't work on Summoner's Rift.