My name is Xpeke and I am the adc of Fnatic, for proof you can googl xpeke and see ho i am.

Lie, garen is the most bos character in the game, and he is as good as adc as any other good character, specily if you folow this build.

Ok, so yu mite thinc , why garen fir adc if he is not range? yes he is range. In the runes you get cd, why? Because garen R is a good ranged, so he can be adc.
you can also use the smite to get early gold from farm
You also need a muramana for more mana since he has early game mana problems.

Bonetooth neclace is a must, since it makes him snowball a lot.
Also, if you do good (which you will) you will get focused - barrier op

Pros:
Good lanning.
Build in cc W

cons:
Skillshot dependent
Mana problems early game