So full tank Garen hits his R for 1200 true damage

He did that with magic damage before, and still does that to 4-5 members of the enemy team.

Now, if this is just a Garen QQ thread I'll say this; your post is shit and doesn't do anything to support discussion.

If it's not a QQ thread I'll attempt to answer your question.

Garen does have to build damage. A full tank Garen only works if he is focused every fight, and that isn't Garen winning, that's the enemy team failing. His ratios are good, and his CC is poor against any team that isn't caster heavy. There is really no legitimate reason to build no damage on him.

How much damage you should build is a matter of debate, as good Garen builds range from a one damage item Bruiser to a one/two tank item fighter, but no one who knows anything about playing Garen outside of the very low ranks will say Garen should be built as a tank.

Full tank Garen does no damage whatsoever; the fact that you're even considering it shows you have no idea how Garen works. Garen's always been able to build one or two damage items and achieve great damage, while being hard to kill as he spends the rests of his slots on defense.

Also you do realize that the true damage thing is only active for one enemy at a time, right?

Garen's ult is also the only thing doing any damage if he builds full tank, since it doesn't rely on any AD or AP ratios to deal damage.

Garen players tend to build two damage items, since that is the best way to remain a threat all game while being tanky.

I dont QQ about Garen but i dislike the new trend that Riot is following where some champions dont need to build damage items to deal massive damage and from the other side, defensive items to be extremely tanky. 1200 TRUE damage from a champion that dont have a single item with attack damage is kinda extreme dont you think?