Not sure what you mean by “recommended GAP”. The gap_days are a value that is derived from the data, there is nothing to recommend. What you can play with as an input parameter is the persistence window for building eras. However, we set that to 30 days as a standard. If you want other eras you may want to generate cohorts.
More discussion about gap_days and persistence window you can find here: Where does gap_days come from when building CDM v5 drug_era?.