How to set up recurrent and customized auto reply in Gmail for free
In the search string, you could specify which emails you want to do the auto reply. Normally, you would like to include 'in:inbox' in the search string, otherwise even those emails you sent will trigger the zap, and cause unexpected loops.
Tip 1: you could search by label if you want to do auto replies for certain labelled emails, e.g. 'label:leads'
Tip 2: you could do recurrent email replies, if you also add a date in the search part, e.g. 'older_than:7d' for trigger to work on those email of more than 7 days old.
Limitation: the free Zapier account won't expire, however it could be triggered at most 100 times per month. So in this case, at most 100 auto replies could be sent every month. It's useful if you only need to do a few targeted and customized auto replies every month.