What do the acronyms in "AT 2017 gfo" mean?
You are correct. The AT is short for Astronomical Transient. The follows the year and an arbitary sequence of letters to uniquely identify the event, which is assigned by the Transient Name Server

This is the official IAU designation for Supernova Candidates, however true supernovae will be re-designated "SN YYYYX". The neutron star merger was not a supernova, so retains its AT designation

This is described at (source) with more details at Transient Name Server (TNS)