GCOS is the collective name for a group of mostly-unrelated operating systems from GE, Honeywell, and Bull/Atos. GCOSes run on systems ranging from the Level 61 small business minicomputer to large mainframes like the DPS 9000/TA300.
Long-dead OS for the Level 61 system inherited from Bull. Primitive.
Minicomputer OS. EOL'd in Bull's markets in the late 1980s, but a variant is still sold by NEC (under emulation) as ACOS-2.
Minicomputer OS. Multics-like command shell. Sold for both technical/control workloads and business workloads. Killed circa 1991 and migrated to an AIX-based emulation environment, HVX.
Entry mainframe ("midframe") OS. 32-bit, EBCDIC, and has a robust UNIX environment (which also provides services for TCP/IP.) Large but declining customer base in Western Europe, almost none in the US. NEC sells a variant as ACOS-4, running on custom CISC CPUs; Bull's own systems were moved to emulation on x86 and Itanium systems around 2001.
The high end of the GCOS family. Originated at GE circa 1962, runs on 36-bit mainframes (until ~2006) and under emulation on Itanium. GCOS 8 still has customers in both America and Europe. NEC sold a variant as ACOS-6 after inheriting Toshiba's mainframe customer base, but mostly abandoned 36-bit systems through the 1990s and repositioned ACOS-4 as the top end of their product family.
A writeup I did on GCOS 8 history, hosted with permission at linkerror.com