There are two GCOS simulators on Multics. One is a batch simulator and the other is a GCOS time sharing simulator (GCOS TSS).
These simulators simulate GCOS 3, usually just known as GCOS, which was the operation system that ran on the GE600, GE600, L66/L6000/DPS300. The "map355" command used to assemble the FNP source code invokes the GCOS (batch) simulator. The DN355 cross assembler is written in L66 code, so Multics just needs to emulate the system calls. The GCOS emulator copies the GCOS program to a segment, and executes the segment in BAR mode with an offset of 0; the GCOS application does OS calls with the MME (Master Mode Entry) instruction. The GCOS emulator intercepts the MME calls with Multics exception handler, deciphers what the call wants, does it, and continues the GCOS program.
GCOS users at AFDSC (Pentagon) used to run their GCOS batch jobs under the GCOS simulator on Multics since they ran faster there than under native GCOS!
There is a also the GCOS TSS subsystem which is an interactive programming environment supporting several languages (Multics includes the BASIC and FORTRAN runtimes). Sadly, there remain some simulator bugs that are causing some failures under TSS; and lacking the source code for TSS, it is proving to be an intractable issue.
The GCOS simulator is documented here: Multics GCOS Environment Simulator - AN05-02A