The reason Snow White triggers Erlang Shen’s damage is because she clears out all ailments and buffs from both the enemy and allies. Enemy ailments is the key to triggering Erlang’s extra damage.
Easier would be to fire another attack down on top of it and that triggers.
Gazelle clears all ailments and buffs from allies which is why she does not trigger Erlang’s extra damage.
Search the Snow White threads to read about her (hint you can use the search function, I have added the two links below to save you time):