According to u.gg, Garen has a much higher winrate as Rammus and Amumu, a much higher ban rate than either and essentially the same winrate:
https://u.gg/lol/tier-list?rank=overall
Why are Rammus and Amumu getting nerfed but he is going untouched?
We see Amumu and Rammus above 54% with Garen at 53%
Based on our system of thresholds (meant to keep things consistent and remove bias) this leads to nerfing the first two and not Garen this patch.
Current plan is just a partial revert of 10.4 buffs on Amumu and Rammus, so I don't think they will be in a bad spot after 10.5.