Air carrier Captain reported being notified during pushback of Hazmat that was loaded with documents that required the Captain's signature. The flight was tugged back to the gate where the Captain signed the document and departed safely.

Narrative

Was Notified of hazmat during pushback. I started my preflight duties and was looking through the ACARS messages. The flight indicated NO HAZMAT on board. The flight crew performed their regular preflight duties without any incidences. During pushback; the Lead Ramper said there was biological substances onboard the flight. I question if it was HAZMAT and he said yes. Since I needed the paperwork thinking HAZMAT was boarded at the last minute; I asked if he could give the paperwork through the cockpit window in order to sign and keep the copy. He informed me he couldn't reach that high. Flight crew decided to performed a Gate return and the Ramper tugged us back to the Gate. Once at the Gate; the Lead Ramper informed me of biological substances. The Ramper stated according to his operations memo; I needed to sign the paperwork. Looking at the paperwork; the number was UN3373. According to the FOM; biological substances (UN3373) doesn't need pilot notification as long as they are properly labeled even as gate checked bags during domestic flights. Since the Ramper said biological substances; I assumed the package was properly labeled. Yet the lead Ramper said I needed to signed the form. I signed the form and the flight continued without incident.