Heroes Rise: Destiny Calling - S3-E21
Continuity mistake: As Bullock and Gordon face Fish Mooney in the tunnel, just as Fish says "Uh, no'", there is an angle change and Gordon suddenly goes from holding his gun in a high, two-handed grip, to a low, one-handed grip. (00:13:15)
Answer: "The Merc" was a warehouse like store with aisles that sold mainly weapons, machetes, AK-47s, C-4, barrels of acid, etc. In s02e05, "Scarification", it was shown being raided by GCPD.
Bishop73