GECOS was originally the General Electric Comprehensive Operating System. It was a clone of System/360 DOS that emerged from GE around 1970.

Some early Unix systems at Bell Labs used GCOS machines for print spooling and various other services; the field added to /etc/passwd to carry GCOS ID information was called the `GECOS field' and survives today as the pw_gecos member used for the user's full name and other human-ID information.

When you use adduser, and you enter your real name and room number, you are entering GECOS data. If you want an automated adduser(8), then specify --gecos "Some Data" in your script.

See JargonFile:GCOS