The GECOS-II operating system was developed by General Electric for the 36-bit GE-635 in 1962-1964. Contrary to rumour, GECOS was not cloned from System/360 [DOS/360?] - the GE-635 architecture was very different from the IBM 360 and GECOS was more ambitious than DOS/360.
GE Information Service Divsion developed a large special multi-computer system that was not publicised because they did not wish time sharing customers to challenge their bills. Although GE ISD was marketing DTSS - the first commercial time sharing system - GE Computer Division had no license from Dartmouth and GE-ISD to market it to external customers, so they designed a time-sharing system to sell as a standard part of GECOS-III, which replaced GECOS-II in 1967. GECOS TSS was more general purpose than DTSS, it was more a programmer's tool (program editing, e-mail on a single system) than a BASIC TSS.
The GE-645, a modified 635 built by the same people, was selected by MIT and Bell for the Multics project. Multics' infancy was as painful as any infancy. Bell pulled out in 1969 and later produced Unix.
After the buy-out of GE's computer division by Honeywell, GECOS-III was renamed GCOS-3 (General Comprehensive Operating System). Other OS groups at Honeywell began referring to it as "God's Chosen Operating System", allegedly in reaction to the GCOS crowd's uninformed and snotty attitude about the superiority of their product. [Can anyone confirm this?] GCOS won and this led in the orphaning and eventual death of Honeywell Multics.
Honeywell also decided to launch a new product line called Level64, and later DPS-7. It was decided to mainatin, at least temporarily, the 36-bit machine as top of the line, because GCOS-3 was so successfull in the 1970s. The plan in 1972-1973 was that GCOS-3 and Multics should converge. This plan was killed by Honeywell management in 1973 for lack of resources and the inability of Multics, lacking databases and transaction processing, to act as a business operating system without a substantial reinvestment.
The name "GCOS" was extended to all Honeywell-marketed product lines and GCOS-64, a completely different 32-bit operating system, significanctly inspired by Multics, was designed in France and Boston. GCOS-62, another different 32-bit low-end DOS level was designed in Italy. GCOS-61 represented a new version of a small system made in France and the new DPS-6 16-bit minicomputer line got GCOS-6.
When the intended merge between GCOS-3 and Multics failed, the Phoenix designers had in mind a big upgrade of the architecture to introduce segmentation and capabilities. GCOS-3 was renamed GCOS-8, well before it started to use the new features which were introduced in next generation hardware.
When Honeywell decided in 1984 to get its top of the range machines from NEC, they considered running Multics on them but the Multics market was considered too small. Due to the difficulty of porting the ancient Multics code they considered modifying the NEC hardware to support the Multics compilers.
Several versions of transaction processing were designed for GCOS-3 and GCOS-8. An early attempt at TP for GCOS-3, not taken up in Europe, assumed that, as in Unix, a new process should be started to handle each transaction. IBM customers required a more efficient model where multiplexed threads wait for messages and can share resources. Those features were implemented as subsystems.
GCOS-3 soon acquired a proper TP monitor called Transaction Driven System (TDS). TDS was essentially a Honeywell development. It later evolved into TP8 on GCOS-8. TDS and its developments were commercially successful and predated IBM CICS, which had a very similar architecture.
In the late 1980s Bull took over Honeywell and Bull's management chose Unix, probably with the intent to move out of hardware into middleware. Bull killed the Boston proposal to port Multics to a platform derived from DPS-6. Very few customers rushed to convert from GCOS to Unix and new machines (of CMOS technology) were still to be introduced in 1997 with GCOS-8. GCOS played a major role in keeping Honeywell a dismal also-ran in the mainframe market.
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.