PostgreSQL is pronounced post-GRES-Q-L, and the official FAQ says saying just Postgres is fine.
The name is a timeline: POSTGRES at Berkeley in 1986, Postgres95 in 1994, then PostgreSQL in 1996 with version numbers restarting at 6.0.
In the PostgreSQL 18.1 documentation the full name appears 2,918 times and Postgres only 18, so people say Postgres and write PostgreSQL.